Most Helpful Girl
Unfortunately there's not much you can do. People don't tend to listen to their friends about their significant others until it's too late, they're broken up, and they're looking at you saying "oh my god why didn't I listen to you!!!" It's best to say the minimum. Complain too much about her and he will start to resent you. Or worse, he will tell her, she will resent you, spin all sorts of shit and negativity on to you, and you'll barely see your friend at all until the relationship is over.
When he's having problems be supportive. Trash the SITUATION and not HER. If you retain any of what I'm saying its this! Instead of "oh man you can't come to the party? Your girlfriend is such a bitch you really need to dump her already" say "aw man it sucks she won't let you come. It's going to be a great time and I was looking forward to seeing you there"
In the meantime is there some sort of compromise? Can she go to the parties with him? That way at least you get to see your buddy for a little while?0THIS IS NOT RELEVANT ANYMORE
Most Helpful Guy
Sadly there's not much you can do.
Right now, the best thing you can do, is to keep calling her out on such behavior or pointing it out to him.
Make sure that he's aware that not only is her behavior inappropriate, she is a rare and crazy person that makes you wonder why he even bother. Perhaps pointing out how much she really forbids him that he enjoys.0THIS IS NOT RELEVANT ANYMORE