So pissed

I am so close to blowing up at one of my best friends it isn't even funny. We have a mutual friend who is her best friend in the world. She has a boyfriend, but she has been leading our friend on behind boyfriend's back. My friend told her how he really feels today, and she said "Don't put me in this position" when it's her own fault for leading him along. He's starting to think that at this point she's using him to get the new Project Diva F game. I love her dearly, but this is really pissing me the hell off. Your opinions?