I say she should dump him.
Wait, this isnt a relationship advice thread -- darn!
I haven't seen it happen either, but, then again, we never would invite anyone like that. I feel sorry for your friend. I can't predict how her BF will react, since I only really know how to predict how good people will act/feel. The starry-eyed sparkle pony in me says that anyone can be converted or at least gain some insight. I wouldn't rule out the possibility.
I will say that you should try very hard to get your friend away from him for at least 1 or 2 days/nights so she can see how amazing the experience can be when surrounded by positive people. Maybe that will open her eyes to his issues and give her something to look forward to when she comes back the following year without him
I would also echo all of Eric's advice.