We can't prove that somebody named Aristotle wrote a book called "Ethics."
A 2000+ year old tradition has handed it down that this book can be ascribed to this famous philosopher, but there's no hard "proof" and certainly no autograph copies, so that's no better than the case for the Bible. What the theist who argues these points can apparently never understand is that because books of antiquity like these make no claims to the supernatural, we have no need to devote the endless, torturous arguments/defenses to its authenticity the way we have to do with the Bible. Our default position is to accept tradition and ascribe authorship to Aristotle -- and if it were written by one of his students, or tampered with over the centuries, it just isn't that important because nobody is worshipping Aristotle as a God. Nobody is making the "faith claim" that if "Ethics" wasn't written by Aristotle, it would destroy their whole little doctrine-centered universe. It's the ideas that matter -- who wrote it is unimportant.
The Holy Books can't make such a claim -- their entire value is bound up in the supposed "divine revelation" that "prophets" received.