I’m with Bill too.

All you can do is to be there for him and make him feel comfortable enough to open up to you.
I understand your concern because he’s your friend and has issues but maybe, he will grow with this relation with this girl.

I do agree with you that a person (man, women, it doesn’t matter) has to treat other with respect so as close and open as you can guess to him, the better for him to trust you and listen to your opinion.

Good luck to you and your friend.