I see that you have gotten some great advice since last night, Plenny. I think the biggest thing right now is what has already been mentioned: your friend is going to treat you like the enemy because she feels her drinking is being threatened. You are not doing anything wrong, and have nothing for which you should feel guilty. This is just normal alcoholic modus operandi, all par for the course.
Detachment will probably be very good for you remain friends with this girl, but still protect yourself and your boundaries. It can't hurt to state what your boundaries are, but that you are still there for her. Let her know that, should she ever decide to seek help, that you will be there for moral support. That you only want the best for her, but that you must make sure that you stay healthy, and being around her when she is drinking is not acceptable. You are not abandoning her, but simply loving her from a distance. This is so hard, but we will always be here to answer your questions and provide support.