My friend is going down a bad path.....?

my best friend of 15 years suddenly doesnt want to hang out with our group of friends anymore, but chooses to hang out with the burnouts of the school..who she thinks are the coolest. we're still friends but she is changing and making really bad choises... her boyfriend has done drugs and drinks, and now i find out that she fooled around with him at a party...after dating him for a week. she snuck out of her house to go see a guy who was on house arrest for doing pot...she cusses at her parents and has an "i dont care" attitiude about everything.. ive known her and been her best friend since we were babies and its so hard for me to stand by and do nothing...she also doesnt have a lot of commen sense, and im afraid this will get worse and she'll wind up pregnant at age 16...how can i help her see that shes heading down a bad path without losing her completely as a friend?