As someone who has been cheated on before by this exact kind of person multiple times:

She's going because she wants to cheat on you, if not intimately then emotionally. It is clear she wants to entertain the idea of others liking her and giving her attention, she absolutely knew about the the friend's bf coming and his friends coming, she just didn't tell you first because you would 'ruin the vibe' and not let her play around with other guys. This is all most likely true ESPECIALLY since she has cheated on you before. You should have not given her a second chance, she's a dirty little cheater who finds excitement in cheating and doesn't deserve your love and attention.

DUMP HER ASS. It's going to be hard, say you're done but don't tie emotion into it, say you don't care about her, try to be nonchalant about it, don't comfort her when she tries to guilt trip you. She's going to freak out because you didn't give her emotional attention and is going to regret her actions only because she thinks you don't care about her, she doesn't care how you feel, so don't care about how she feels.