Help! She's a monster now because she thinks she's prettier than than everyone!?

Ok this girl is not even my friend but this has got to stop! Shes acting like a total snob by gossiping behind everyones back including me and she has been using my friend and making fun of her when shes not looking. She think she is so pretty but really she's a total chub and she wears the same clothes everyday, her face is all werid and she thinks shes popular her hair is always a mess and she looks like a fat slut. Please help I really want to help my friend. I never said anything as mean as this before but no offence but it really needs to be said. What should I do?