My best friend is underage and going off the rails...?

my best friend is only 13 and has had a hard time at school ever since i met her. she got into alot of trouble with naked pictures she carelessly sent to various "boyfriends" which were sent around loads of schools. she was never really popular within school and had the reputation as a bit of a slag but now its like she'll do anything to become popular. she told a girl in a more popular group that she has with her then boyfriend and whilst on a school holiday the girl told everyone and openly admitted she pretended to be friends with my best friend just to get gossip. my best friend is now trying to cosy up all in their group and was smoking and drinking with them just to fit in. its like she has no self respect to go with people who openly backstabbed her. today at school she got into a "fight" (more like her trying to look hard) with a girl in an older year. we go to private school and things like that dont happen very often so now everyones all hyped up thinking its the most amazing thing to ever and all attentions on her. i still really love her and i can tell she still wants to be my friend but i hate to see her faking it in this group with a girl who a couple of weeks ago had her in tears. i never really understood peer pressure before but now i really do, everyones smoking and drinking and its like if you dont do those things your not in with everyone else. what should i do or should i just stick it out? i know its not wise, but should i start too?