How can you help a friend who is making a lot of bad choices?

I've been friends with *Kerry for many,many years. She is like a sister to me. Recently she has started hanging out with this new girl, *Staci who is a complete idiot! *Staci smokes pot and already has a drinking a smoking problem. *Kerry has started to act just like her. They go out and drink every weekend. They nearly got into a wreck because they were drinking and driving. I can't just tell her parents about it, she would never forgive me, even though I'm trying to help. This new girl's mother lets them drink and smoke. I can't tell a counselor, because I'm home schooled.And please don't tell me how stupid my friend is, I've heard that enough. My mom has warned my friends mom about how dumb *Staci is, but she says she trusts *Kerry to make the right decisions. We are only fifteen.

I would tell her parents. Yes, you will probably lose a friend. You may very well also save a Girl's life. Is that not a good trade-off? Think about it.