Ugh. i hate this girl!?

Okay so I go to school with this girl and she is one of the worst people in the world to be with (in my opinion)! She flirts with every other guy EXCEPT her boyfriend, which includes my own, and she dresses like a skank, and she has really nasty back-hair. She thinks she's so great, and she thinks she has so many friends, when really she is so mean to people that she creates every one's enemies and her own enemies. I really want to say something to her, but I don't want to be mean. WHAT SHOULD I DO?!