The sad thing about this is that no one can MAKE her stop doing these things until she's actually ready. I worked in the liquor industry for 13 years and I can't even count the number of times I've seen people "go to treatment" only to be back in the store in less than a week. It's obvious that you're a caring person who is genuinely concerned about the danger that your "friend" is putting others in, but realistically if her parents are ignoring/supporting this she has even less reason to try to change. I would send them a heartfelt letter, or email, or whatever allows you to maintain distance from the situation, and then follow through on your plans to cut ties.

Based on my experiences with police in situations like this they won't be of much help, but if reporting the incident helps you to feel better about the situation then definitely do it. Worst case scenario is probably them ignoring it. All you can do at this point is try to limit the damage to yourself. Good luck OP, I know how hard it is when a former friend chooses substances over you and everything else.