The claim is that there are two people, Lazar Wiesel and Elie Wiesel, and that Elie Wiesel stole the identity of the (presumably dead) Lazar Wiesel after Lazar Wiesel had written a book in Yiddish, called Un di velt hot geshvign. Elie Wiesel then used that as a basis for his own book, Night.
But most claims made that is supposed to support this assertion are simply incorrect or inconsistent.
For example, it is claimed that Lazar Wiesel at Buchenwald Concentration Camp was born 1913, and arrived there with his brother Abraham born 1900. They also claim that Lazar's signature is different from Elie's. The evidence for this is a form filled out after the liberation with Lazar Wiesel's signature on it.
However, that form sets Lazar Wiesel's birth date to 1928 in Romania, the same as Elie's claimed birth date and country. So why is there an detainee card for a Lazar Wiesel that claims he was born 1913 in Hungary? And why is that detainee card showing he came to Auschwitz in 1945, when Elie claims he came there in 1944?
Well, either they are two different Lazar Wiesels, one Hungarian born 1913, and one Romanian born in 1928, being released from Auschwitz, or one of the forms is incorrect. The number on the detainee card is hard to read. Elie Wiesels number is A7713. The number of the detainee card is A77??. Looks more like A7798 to me, but it's hard to say.
But if it's the elder Lazar's signature on that form, then it's the elder Lazar that is stealing the identity of the younger Lazar, not the other way around. That would invalidate the whole claim.
It is also sometimes claimed that Elie Weisel wasn't a victim of the Holocaust at all, but took over Lazar Weisel's identity after he wrote the book. But that then invalidates all the arguments that the "real" Lazar Wiesel, the actual author, was Hungarian and born in 1913. In that case that has to be a third Lazar. Another possibility is that Elie, that was 15 when he was arrested, simply lied about his age, so that he would be interred with the adults, instead of sent directly to be killed as a child. This possibility does not seem to have struck any of the people claiming he is a fraud. On the other hand it seems unlikely that they would have believed that he was 31.
It is also sometimes claimed that Elie Wiesel doesn't look like the person claimed to be Lazar Wiesel in a photo from the concentration camp. A lot of the evidence of this comes from pictures just after the war when Elie Wiesel was 16 or 17 years old. These do look like somebody who is 16 or 17 and is obviously not somebody who is at this point 32-33. Indeed all the conspiracy sites claim that these pictures show the real Lazar Wiesel, and that he is 16 or 17. That is inconsistent with their claims that the real Lazar Wiesel is born in 1913.
The claims that this young man doesn’t look like Elie Wiesel are obviously subjective. Personally, I think that's hogwash. The person in the photos from just after the war looks very much like Elie Wiesel, meaning that he not only was the same age as Lazar Wiesel, but also looked remarkably similar. It is equally absurdly claimed that the photos of a 15-year old Lazar before arrest looks nothing like the Lazar shown in a photo from Buchenwald, and identified as the real Lazar by another person in the photo (although he claims Elie Wiesel is not that person).
Then the only evidence remaining is that his signature has changed since the war. That's not much evidence, especially considering that there is not only many years in between the two signatures, but that he also has become famous and gets to sign a lot of books.
And if there was a Lazar Wiesel that wrote the original book, didn't anybody meet him or know about this? Did he write and publish that book without telling any friends or family about it? Why has nobody come forward and said "I knew Lazar Wiesel and he is missing"?
Also, if Lazar Wiesel is not really Elie Wiesel, should he not have any friends or family that can identify him and say what Lazar's real name is? Unless it also is Eliezer Wiesel. In which case we have an Eliezer Wiesel born circa 1928 that takes over the identity of another Eliezer Wiesel also born in 1928, with whom he shares such a remarkable facial similarity that nobody notices.
And that, frankly, is quite absurd.