Talk to him. See if you and him can spend a night out just doing what you two used to do, and if his girlfriend asks about it being a date, tell her that you're his friend and he needs friends as well. If she was so confident about the state of the relationship she shouldn't be jealous, and she's jealous right now.
Anyways, just spend the night or whatever trying to figure out what's going on. Apologize to him, but don't try to like her. You don't have to, just be happy for him. He's your friend, and you've clearly stated she's not. Just don't hurt him. He's going to want a friend when the relationship ends.