how do I break this toxic relationship

Okay so have any of y'all out there been really good friends with a girl, then she got broken up with and went totally crazy? So I have this friend, her and her man we're dating for a year, she lost her virginity to him and she thought they were going to get married etc. well they broke up in January, bc she was very clingy and he just needed time to be a guy and whatever, so this girl just started doing drugs, acid, Molly, and just not good stuff, then she got mad at me when I was still frinds with him ( we were friends before they started dating) anyways, she got over it and we were still friends, then she banged his bestie, and this other guy, but still has the nerve to wear the necklace he got her, and how much she loves him and will always love him bs. I mean I know break ups hurt but I have been more than nice to her letting her shit all over me, how do I break this toxic relationship, I'm just so over her lying to me and going behind me my back and I'm done with it, send help plz!!