I'm going to warn this girl about her date.
What are you planning on saying?
I will tell her that her boyfriend is a fucking autistic asshole.
While normally it is not good to interfere in a person's relationship, it is not good to stand by and not speak up when you know somebody is going to suffer. It will safe her a lot of headaches.
I know, I'm going to do this with good intentions.
Please tell her privately you could create a dangerous situation if you do it in front of him.  And you know it isn't nice to make fun of peoples disabilities and it isn't an excuse for their bad behavior.