As hard as it is to hear, we have no control over the choices and decisions that other adults make. I know that you are worried about your friend and think that for her sake you need to do all you can including "telling on her" to her mother about everything that is going on. What this might do, unfortunately, is cause her to resent both you and her mother and never speak to either of you again. I can't believe that is the outcome you want for your friend.
Sadly, what Suki said is exactly correct. You have no control over this situation. We, the friends and family of alcoholics and addicts, learn something called the 3 C's.
We did not cause it (the drinking or drugging)
We can't control it (not matter how much begging, pleading, threatening, etc. we do)
We can't cure them (the addict)
The same goes for our relationships with the enablers. We can't make someone stop enabling the addict. They are grown ups and must come to this conclusion on their own...as frustrating as that is for us. We can only control ourselves.
Basically, you can't draw a line for someone who is not willing to draw it for themselves.
Hugs to you and your friend. HG