I ended a longterm friendship over my friend's destructive behavior. (She wasn't using substances, but was engaging in manipulative, unkind behaviors. She hurt people in her own family.) The hardest thing is the world - and something I am glad I did - was to tell her what it was that I objected to. I just told her I was observing X, Y, & Z and it reflected poorly on her and that's not what I look for in a friend.
The reason I bring this up is that your friend might be so caught up in this lifestyle that she doesn't see the bad in it. If you fade from her life, w/o comment, she will never have the gift of your perspective. If you love your friend, IMHO - you should love her enough to suffer the discomfort of a five minute conversation. It might be a little jolt to motivate her to straighten up. Heaven forbid, but if something tragic happens in this family at least you can say that you tried.
You will be in my thoughts. I wish you every strength.