She even wrote that she will try to find a cure for him.
His behaviour and response (the M letter from the typewritter) practically ruined Mary's life (cancels the book, starts to drink, almost commits suicide, etc.).
He had specifically said in an earlier letter to her that he did NOT consider his Asperger's as something that he wanted be "cured" of. It was part of him, an aspect of who he was. Studying and learning about his disability was one thing, but when Mary said she was hoping to discover a cure for it, it was as good as telling him that she rejected him, and regarded him as damaged and deficient.
Amen to that (I, too, have AS). I could sympathise with Max, although I'm a strange case, since if somebody offered to make me "normal" for 24 hours, just one day, then I'd probably take up the offer, just to see what life is like for a "normal" person.