Betrayal...?

Hey, guys..umm, here's the story, one of my friends recently came back into "exsistance", no one knew where he was, and he wasn't returning any phone calls, and I was really worried about him, because he's like, my best friend (brother from another mother.). And when he finally does come back, I'm glad to see he's okay, and I invite him to hang out with me the next day. But, later that night, I find out he was trying to get with my lady. And I got very upset with her and my friend. So, I don't really know what to do right now, I got to face my friend tomorrow and don't know how to react to him, I feel so betrayed, I didn't think he had the capability to do something like this. And my girl wasn't telling me about any of it, or stopping it, so, I'm really not feeling her anymore. I just want them to both go away, but since that's not going to happen.......any advice?