My Best Friend is a w*ore and she keeps lying to me!?

My Best Friend is in a relationship and has been for the past year and a half (on/off, because he's cheated on her more than twice). I know he's no good for her but she can't seem to see it, or can she? Recently, she's been having some "revealing" webcam sessions and heated chats with various other guys, not that she told me. After I heard rumours, I then confronted her where she furiously denied it any involvement with the guys other than innocently chatting, so I resorted to checking up on her through her fbook inboxes. There I saw conversations that proved the rumours correct and that she had lied to me. To my face. I'm worried about her as I think she could be sleeping around also. She know her boyfriend cheated on her but still refuses to break up with him, is it because she feels guilty? The relationship is not healthy for either of them (It doesn't help that he's a spineless d*uche creep who cries everytime he's confronted with the truth). Shes on a downward spiral to destruction and I'm worried about her, what do I do? Should I confront her again, knowing what I know?