Actually, there were many things that he got unmistakably correct as well that were a total shot in the dark, so it's not a cut and dry argument that the Book of Abraham is incorrectly translated or altogether a fraud.
In fact, if you studied and understood the english translation itself and dissected topics and passages, you would find that the literature is unmistakably of hebrew origin. Yes, hebrew, not egyptian. The descriptions of visions and such are unmistakably described in hebrew manner of speaking, and uses ancient hebrew modes of expression that are completely absent in english. Since it was supposedly written by a hebrew, writing in egyptian characters, this makes sense.
Equally, ancient hebrew poetic expressions are common in the Book of Mormon. Chiasmus mode is found in the book of mormon, and an entire chapter is written in it, which happens to be one of the longest and most complex examples of a chiasm ever known. Since the Book of Mormon people were a branch of migrated hebrews, this makes sense.
There are things people like to point out that would suggest that he was a fraud. The fact is, there are many many things that he did that proved him legit as well. His scriptural writings, however heavily criticized, have actually held up to the criticism and there are also many things that prove that his writings/translations are unmistakably inspired.