Short answer
The Buddhist scriptures give both answers. Devadatta was a monk who, the Pali rule book says, tried to kill the Buddha and split his monks. In the Pali discourses the Buddha says Devadatta is going to hell, there to remain for an aeon, an immensely long age, irredeemable. In the Lotus Sutra, one of the best-loved Mahayana scriptures, the Buddha calls Devadatta his good friend and says he will one day become a Buddha. The two scriptures paint opposite pictures of the same man, and both claim to be the Buddha's own words. The Lotus Sutra's own translator wrote that this story had been profoundly altered.
Full explanation
Who Devadatta was
The Cullavagga, part of the Vinaya, the monks' rule book, tells how Devadatta plotted against the Buddha. He climbed the Vulture's Peak and hurled down a great rock to kill him. Two peaks stopped the rock, and a splinter made the Buddha's foot bleed. When Devadatta set out to split the community of monks, the Buddha warned him that whoever breaks it up is boiled in hell for an aeon.
“And Devadatta climbed up the Vulture's Peak, and hurled down a mighty rock with the intention of depriving the Blessed One of life. But two mountain peaks came together and stopped that rock, and only a splinter falling from it made the foot of the Blessed One to bleed.”
“Whosoever, O Devadatta, breaks up the Samgha, when it is at peace, he gives birth to a fault (the effect of) which endures for a kalpa, and for a kalpa is he boiled in niraya.”
In hell for an aeon, irredeemable
The Numbered Discourses are a collection in the Pali canon, the scriptures of Theravada Buddhism. That is the Buddhism of Sri Lanka and Southeast Asia. In them, speaking not long after Devadatta had left, the Buddha says what overcame him: gain, fame and honour, and their losses, corrupt wishes and bad friendship. So Devadatta is going to a place of loss, to hell, there to remain for an eon, irredeemable.
“Overcome and overwhelmed by eight things that oppose the true teaching, Devadatta is going to a place of loss, to hell, there to remain for an eon, irredeemable.”
A good friend and a future Buddha
The Lotus Sutra is a Mahayana scripture, from the branch of Buddhism found mostly in East Asia. In it the Buddha tells of a past life in which he was a king who served a seer to learn the law. That seer, he says, was Devadatta. "Devadatta was my good friend. By the aid of Devadatta have I accomplished the six perfect virtues." He then declares that after countless ages Devadatta will become a Buddha, named King of the gods.
“For it was this Devadatta himself, the monk, who at that time, at that juncture was the Seer. Indeed, monks, Devadatta was my good friend. By the aid of Devadatta have I accomplished the six perfect virtues (Paramitas).”
“I announce to you, monks, I declare to you: This Devadatta, the monk, shall in an age to come, after immense, innumerable Aeons, become a Tathagata named Devaraga (i. e. King of the gods), an Arhat, &c., in the world Devasopana (i. e. Stairs of the gods).”
Traces of alteration
H. Kern, who translated the Lotus Sutra from Sanskrit in 1884, wrote that the whole story is so different in language, style and spirit from anything else in the Lotus, and so profoundly altered, that almost every word must be taken in another sense than it first had. He spoke of those who modified the ancient tale. Judging by their metre, he called its verses "one mass of corruption". He also noted that a later hand had added a note in the margin of a manuscript, meant to give the tale a Buddhist tinge.
Kern writes that "the whole story, so different in language, style, phraseology, choice of words and spirit from anything else in the Lotus, has been so profoundly altered that almost every word must be taken in another sense than what originally was attached to it", and speaks of "those who modified the ancient tale". In his notes on the verses that tell it, he writes that "the text of these verses is one mass of corruption, as is proved by the repeated offences against the metre", and of the verse that ends it that "the traces of alteration are so clearly visible that it is not necessary to point them out". He also notes, on one word of a verse, that "a later hand has added a marginal reading", "obviously intended to give a Buddhistic tinge to the tale".
Both cannot be the Buddha's word
The Pali texts and the Lotus Sutra paint opposite pictures of the same man. In one he is overcome by corrupt wishes and bad friendship, and tries to kill the Buddha. In the other he is the Buddha's good friend, by whose aid the Buddha gained his perfections. A Buddhist who holds both must say that one of them is not the Buddha's word. And the scripture with the kinder picture is the one whose own translator found the story altered.
“Overcome and overwhelmed by eight things that oppose the true teaching, Devadatta is going to a place of loss, to hell, there to remain for an eon, irredeemable.”
“And Devadatta climbed up the Vulture's Peak, and hurled down a mighty rock with the intention of depriving the Blessed One of life. But two mountain peaks came together and stopped that rock, and only a splinter falling from it made the foot of the Blessed One to bleed.”
“For it was this Devadatta himself, the monk, who at that time, at that juncture was the Seer. Indeed, monks, Devadatta was my good friend. By the aid of Devadatta have I accomplished the six perfect virtues (Paramitas).”
Kern writes that "the whole story, so different in language, style, phraseology, choice of words and spirit from anything else in the Lotus, has been so profoundly altered that almost every word must be taken in another sense than what originally was attached to it", and speaks of "those who modified the ancient tale". In his notes on the verses that tell it, he writes that "the text of these verses is one mass of corruption, as is proved by the repeated offences against the metre", and of the verse that ends it that "the traces of alteration are so clearly visible that it is not necessary to point them out". He also notes, on one word of a verse, that "a later hand has added a marginal reading", "obviously intended to give a Buddhistic tinge to the tale".
One Book without discrepancy
The Qur'an invites its readers to consider it with care. Had it come from anyone other than God (﷾), it says, they would have found much discrepancy in it. It also says that God (﷾) sent it down and will guard it.
“Do they not consider the Qur'an (with care)? Had it been from other Than Allah, they would surely have found therein Much discrepancy.”
“We have, without doubt, sent down the Message; and We will assuredly guard it (from corruption).”
The usual answers
Qur'an 2:106 — Surat al-Baqara (The Cow), verse 106
“None of Our revelations do We abrogate or cause to be forgotten, but We substitute something better or similar: Knowest thou not that Allah Hath power over all things?”
Cited for
The Qur'an also replaces earlier verses with later ones.
The response
The Qur'an itself says that God (﷾) replaces a revelation with one better or like it. That is one Lawgiver changing a ruling within one Book, in the Prophet's (ﷺ) lifetime, and saying so. Devadatta's fate is not a ruling but a fact about one man. Two scriptures that both claim to be the Buddha's word give opposite answers about it.
Where it lands — A change of ruling announced in one Book is not two books disagreeing about a fact.
Sahih al-Bukhari 4987 — Kitab Fada'il al-Qur'an (Virtues of the Qur'an), hadith 4987
Anas reports that Hudhayfa ibn al-Yaman, alarmed at how the soldiers of Syria and Iraq differed in reciting the Qur'an, urged Uthman to save the community before it differed over its Book as the Jews and Christians had. Uthman borrowed the collected sheets kept by Hafsa, had Zayd ibn Thabit and three men of Quraysh copy them, telling the three that wherever they differed with Zayd they should write in the dialect of Quraysh, in which the Qur'an was revealed, returned the sheets to her, sent a copy to every province, and ordered all other written Qur'anic material to be burned.
Cited for
The Qur'an had different readings too, and Uthman had other copies burned.
The response
When two Companions recited a surah differently, the Prophet (ﷺ) heard both and said the surah was revealed that way, so every reading came from him. Uthman had copies made from the sheets gathered under Abu Bakr and sent one to every province. He had other written copies burned, so that one text would unite people who were differing over the Book. No reading makes one man both damned and a hero. The page "How was the Qur'an preserved?" sets this out.
Where it lands — The Qur'an's readings all go back to the Prophet (ﷺ). The two Buddhist scriptures disagree about who Devadatta was.
Perspectives
Mahayana Buddhist
The Lotus Sutra gives the Buddha's highest teaching: every being, even Devadatta, will in the end become a Buddha. An aeon in hell is long, but it is not forever.
An aeon in hell is long but not forever. After it, Devadatta can still become a Buddha, so the two scriptures agree.
Rebuttal
The Lotus Sutra does not speak of a man saved after hell. It calls him the Buddha's good friend, by whose aid the Buddha gained his perfections. The Pali rule book has that same man hurl a rock to kill the Buddha.
The Pali texts give the teaching suited to beginners. The Lotus Sutra gives the final truth, that there is one vehicle to Buddhahood for all.
Rebuttal
Then the Buddha told his monks something about Devadatta that was not his final word. A Buddhist cannot know which of his statements to trust. And the Lotus Sutra's own translator found the text of this story corrupted and altered.
Theravada Buddhist
The Lotus Sutra is not the Buddha's word. Only the Pali canon is, and it says Devadatta is bound for hell.
The Lotus Sutra is a later Mahayana text, not the Buddha's word. So there is no contradiction in the true scripture.
Rebuttal
Then Buddhists do not agree on what their scripture is. Mahayana Buddhists hold the Lotus Sutra as the Buddha's own teaching, and Theravada Buddhists reject it. Buddhism has no agreed book that could settle the matter. The page "Which texts are the Buddha's word?" sets this out.
What the texts say
The Pali discourses send Devadatta to hell, irredeemable. The Lotus Sutra makes him the Buddha's good friend and a future Buddha.
In the Numbered Discourses the Buddha says Devadatta is going to hell, to remain for an eon, irredeemable.
In the Lotus Sutra the Buddha says Devadatta will become a Buddha named King of the gods.
Kern judged that this story had been profoundly altered, and that its verses were corrupt.
The Islamic perspective
The Qur'an is one Book, guarded by God (﷾), and without discrepancy.
The Qur'an speaks of what would happen had it come from anyone other than God (﷾). People would have found much discrepancy in it.
The Qur'an says God (﷾) sent down the Message and will guard it.