Most Helpful Girl
maintain radio silence no matter what she does, ignore calls and block the number if you can, ignore texts and Facebook requests. act as though she doesn't exist and she will get bored because she isn't getting a reaction from you, she will then leave you alone and find someone else to mess with.
if she says nasty things and your friends tell you, just say oh well then change the subject as she will be hoping you'll get angry and contact her over it
Most Helpful Guy
No drama needed. Become indifferent.
Take your distance from her while she's cheating. She'll turn towards the guy she's cheating with.
She can only mess with your head if you still care about her. Stop that.