Sorry, I know how it is to be blamed for someone else's choices. Gosh, I used to blame myself for it too, and it sucks.
My ex's sister wanted me to break up with him because she felt if he stayed with me, he would get in trouble with the law. She implied that I was the reason he got arrested. Never mind that it was 'his actions' that led to his arrest. No, that was not the issue in her mind! And to think I use to trust and look up to her...YIKES!
You can certainly choose not to deal with her if you'd like, although it might be challenging at times. What he chooses to do is up to him.
Sounds like she's part of his problem, meaning, she doesn't hold him accountable for his actions. It's much easier to lay it on someone else. Unfortunately, you just happen to be the closest thing to him right now. In her eyes I guess that makes 'you' the target. But, if it wasn't you, then it would be someone else she would blame.
Stay strong....you know you're not the issue.