HELP! I need advice...?

I just found out that one of my close friends does drugs [smokes+pot+coke]! I also found out that apparently she's been having , too. I had absolutely no idea... Like honestly, I'm in complete shock right now. I also found out that her family is really messed up and that her mom is never home and her dad is constantly walking out on them, and her older siblings are apparently bad kids too... We're both sopres in high school and I really can't believe it. It feels like something out of Degri right now. I'm not going to lie, I am the most goody-two-shoes kid out there and I just don't know what to do. I guess looking back I should have seen the signs [red eyes, runny nose, REALLY dramatic weight loss] but I was blinded by my childhood innocence and naivety. Plus we live in a upper-middle-cl suburban neighborhood, so I haven't ever been exposed to anything of this nature at all. I really want to help her but I don't know what to do! She's really outgoing and funny on the outside but very shy and reluctant to talk about her personal life and like deep stuff which is why I guess I never managed to piece everything together. We've only been friends for about 3 months, but I've gotten pretty close to her. I don't want to abandon her because of this and I want to help her, but I don't know what I can do without having her push me away. What if she [God forbid] overdoses and dies, so I will be left with the knowledge for the rest of my life that I could have done something to change it? Then again, I don't know if I can even look her in the face again because this completely violates every single moral that I have instilled in me. My mind is screaming to get away from her, but my heart is telling me that I should help her. SOMEBODY PLEASE HELP ME! ))))))):