How to stop my 15 year old friend from dressing sl*tty?

ok so it started when she sung at half time at our basket ball game her and her bf sang "just cant get enoungh by the b.e.p" by the switch up part her dress got caught and her undes were shown for a moment and she got lots of attention by the populars and the boys so she started wearing things like a red bustler and black hotpants . she wears glorified underwear now! and now shes gotten into drinking and smoking i mean shes going down hill people are starting to view skye as a cheap slapper to be taken advantage of! when its just me her and toni shes great but around others and shes an attention whore! like now shes wearing booty jeans and a midriff bearing pink tube top