I can see why you are concerned, sonnenschein. Her behavior is chaotic especially compared to the version of her that you knew before. Her habits are self-destructive.

Honestly, she has to want to help herself first before anything can be done. As her friend, you have to decide what being her friend means to you and what you are willing to risk. Personally, I would divulge my concerns to her and let her know she needs help and I want to help her get that help. This can be done by trying to convince her to see a professional. However, I would not enable her self-destructive lifestyle in any way. She would have to find other friends to do that with. Everyone is different and some would not be willing to risk the friendship.

I don't believe you are overreacting at all. You care about her and she is putting you in a very uncomfortable position right now. It appears to me that she is not considering that at the moment but is mostly focused on herself. Again, she is probably struggling with some issues or a disorder and her behavior is a reflection of that. Keep in mind what kind of toll her lifestyle will take on you if you decide to not intervene. She may continue to pressure you to partake in her behaviors and you will continue to worry about her.