Looking after a friend when you're NOT 'the best friend'?

I suggest you talk to her when she's sober. Let her know that you're concerned about her, but that you also don't feel it's right for you to have to be responsible for her if she's causing trouble. Given the suddenness of the change in her behavior, it may be more than just alcohol you're dealing with, so you may have to decide whether or not this is a person you still want to hang with.