My Friend is having sex with her cousin, smoking weed, and drinking often

She's only 15 but her parents don't care. They buy her alcohol and they let the guy sleepover even though they know what's going on. He's 18, so it's illegal for them to be together, they've also had unprotected sex.  He's started to get her to do drugs and she's becoming really mean, we've been friends for over 8 years and I still really care about her even though she's changing.