Oh, PLEASE PLEASE PLEASE do not go back there. Once it works to suck you back in, he'll relapse and it will magically become your fault. Please. For your dog's sake, if nothing else.
Block him. And while you're at it, tell your addict friend that you're going through too much right now to be helpful to her, and then block her, too.
Sometimes you have to come first and now is that time.