When to cut ties?

I'm to the point where I don't know what to do.

A, has been my best friend since I was 15. We have been their for each other through, babies, birthdays, break-up's. She was my rock, the first person I called when I got the news about my parents accident. She has been my cheerleader, and the one person that would tell me how it is no matter how much the truth hurt.
I have been there for her. I was at the hospital for the births of all 3 of her beautiful children. I was there to help all 1 billion 647 times she quit doing pills. I helped care for her kids while she was sick from detox. I was her biggest supporter to quit doing them so that she could be a better Mom to her kids.

But something has changed.

Something far worse than pills.

And I don't know what to do.

The other day I went to A's house. She owed me 20.00 so we were in her bedroom. She dumped her purse out to count all her cash and a clear baggy with this brown stuff fell into the floor. I picked it up. I was like "what is this, and how did you get it". All she said was that she gave some guy a box of sudafed, and he gave it to her, and that it was meth..... She said she was going to try it. She doesn't think she can get addicted to just trying it once. ( Says the girl who can take one pain pill, and spend the next 6 months putting them up her nose).  I flipped out. WTF are you thinking? Do you know what is in that shit? Shit that is NOT safe to consume. That shit is toxic. Have you seen the before and after pictures of Meth addicts? If you're caught with that...  and God forbid I'm here if they catch you, they will take all of the kids, even mine even tho I don't do that shit just for being around you. They will quarantine our babies! That little baggy is a serious felony!

I love her so much. But I think it is time to say goodbye. I can't risk my kids. And I can't sit by and let her self destruct. What do I do? What about those 3 beautiful kids of hers? I love them as I love my own.

I just don't know what to do :(