Boyfriend Crisis! Please Help Me.?

Okay so my boyfriend has recently started a new friendship with this new girl from school. I haven't spent much time with him because, he is always with her. I've tried talking to him about it, but that new girl is acting suspiciously. She always pulls him away when I try to talk to him, starts talking to him when I come near them. I can see her give me the evil eye from a distance, and I caught her getting all flirty with him. I haven't seen him make any moves on her, but i'm getting worried. I confronted him today, when she was gone for like 20 seconds. He says that she jsut needs some friends, and so he is hanging out with ehr for a while. I tell him what's been going on, and he doesn't believe me! What do I do or tell him? I've thought of slapping her, but I don't want to get in trouble like I did with this other girl at school.