You do not have to do either. Playing along with her rules sends her the message that you agree with her behavior. Telling her off would not help in this matter unless you want to completely end the friendship. In this friendship, she essentially calls the shots and you allow her to. If you do not want to drink then you should not go out to drink. If you do not want to eat large amounts of food then you should not be doing this either.
This is an unfair and unbalanced friendship but the fact of the matter is now, this is the dynamic that has been set up by you and you did this by allowing her to dictate your behaviors. When you try to change the rules of this relationship (i.e. telling her that you no longer will engage in her binge eating, etc) she is going to probably be upset with you. She may yell at you or even accuse you of judging her.
If you still want to be friends, you will have to go forth and change the rules, and endure her being upset with you. Go to her and say something like “I really like being your friend and we have a good time when we (fill in the blank), however, I am not willing to binge eat and drink with you any longer. I do not like it, it is not fun and I recognize how unhealthy it is for you and I.” “We can hang out when you want to do (fill in the blank) but I am not willing to hang out with you to binge eat and drink.” The basic premise here is that you want to be honest with her, tell her what you like about your friendship and what you do not like. This helps you to create healthy boundaries with her and put a stop to her taking advantage of you. Recognize that she is only taking advantage of you because you are a willing participant.
By setting healthy boundaries, you may still be able to maintain a relationship that is not all one sided. As far as her eating disorder goes, if she reacts well to your relationship boundary setting, then you may want to mention that you think she needs professional assistance with her eating disorder. It will be up to you to decide if you want and can bring this up to her. You may best know how to proceed on this matter after you see how she reacts to you standing up for yourself. Take care.