Same thing happened to me. Finally, after a year or so I decided to stop letting her get to me and I went to her as a grown up and said "we've had our differences but I am willing to start fresh and accept you as my best friend's girl."
She broke up with him that night.
I like this answer :thumbsup:
Your situation sucks, no doubt. I was in a similar one at one time--it did NOT work to try to make the dude see reason, unfortunately. But eventually, they did brake up and he felt like a total douchebag for how he treated me.
I think that taking the "high road" is the best bet here. Go to her first, and say something like the above. Then go out of your way to do something nice for her ... something you know she'll like. I mean something HONESTLY nice, not some trick to try to get her to be a jerk. If she acts bitchy about it, oh well ... your friend will see that you're trying to make a serious effort.
Arguing with him or her only forces him to make choices. He's spending more time with her--hearing her side of things--and add that to the fact that guys tend to think with their "other head," and the deck is stacked against you.
So be the bigger man, suck it up, and try to be nice. Maybe she'll come around and start being cool--or maybe they'll brake up at some point. Either way, your friend will remember how cool YOU were during this time.