How can I help my friend spiraling out of control?

My longtime friend of 12 years has fallen down a path of drug addiction and drug dealing. Her whole personality has changed, and because she won’t actually tell me or give me anytime to hang out with her, it is hard to say what is going on, but it seems like the worst.

She hangs out with all new people in a very sketchy neighborhood I avoid, the Tenderloin, in San Francisco. Every time I talk to her something is up, this time she was visiting her boy in General because he got his head kicked in. The last time I actually saw her, she was so out of it, she could barely talk and I saw what I thought looked like track marks on her arm.

Anyway, I did confront her about it a couple of months ago. She completely lied to my face and said that she wasn’t on drugs. She doesn’t have time to hang out with just me, and I feel uncomfortable hanging out with her friends, since I think it is dangerous. What are my options here? How should I carry on this friendship? I am having difficulties letting go, as I really miss her friendship.