Well, I've been in that situation alot on both ends. When I fall for someone else, from personal experience, there's nothing that other person can do. But on a happy note, it's usually temporary...
When the person falls for someone else and you're the boyfriend/girlfriend. It's tough. I chose to stay with him even after he cheated on me with this girl. And we worked through it and we were fine. But it was always in the back of my mind, that I tried so hard to be the perfect partner to him and it didn't matter. So that's my little 0.02$
People will do what they want to do. The best way to go about it is be honest with each other and approach it without yelling and if she honestly has feelings for him, maybe you should let her go, she may just as well come back...
"A rattlesnake that doesn't bite teaches you nothing."