How do I tell my girlfriend that I truly hate her best friend?

Her friend smokes weed, drinks (a lot), is very promiscuous with guys ( seriously, it seems like she's with a new guy every time I see her) and she parties way too often. I don't want her to start following the same path as her best friend of 12 years. I don't mind if they hang out if I'm there too, but I don't want her getting too into that stuff because I have a VERY bad history with everything except for being promiscuous. She also has been sober from all drugs for over a year now. I don't want her to have a relapse, not even one. I can ure that she won't slip if I'm there with them. Don't be afraid to tell me that I'm too worried about this, because I want to know if I am.