What to do when a friend abuses alcohol?

My girlfriend is getting insane when she drinks. She was just arrested for not listening to the police’s demands, is rudely mouthy, acts as if she is alone when she is with friends (taking off drunk to walk home in dangerous neighborhood abandoning friends), she is defiant, tells us that she doesnt care “if you dont want to be my friend, fine, this is who I am, im a drunk.” depressed she wants to get wasted, by herself at a bar, and walk home by herself. She refused to get in my car. I finally yelled at her in frustration. Im sick of this! This isnt the girl I knew, something has happened to her and I cant help her. She needs help. She is losing her friends. I want to be her friend but she isnt reciprocating. She refuses to hear me say how important she is to me and that I love her. What do I do? Do I let her go? Im going to tell her that im not hanging out with her if there is alcohol involved. I believe she will decide to not be my friend. She is so defiant. Why does she feel like no one likes her when all we do is show love and support? Why is she pushing us away?