Is there anything I can do to get my husband to set boundaries with his abusive mother, or walk away and let her go to assisted living?

My husband takes care of his 88 year old mother 3 days a week, while our youngest son and his brother and a neighbor help the rest of the time. His mother has always been abusive, in all ways, claiming ignorance, that she was just raised that way, but even after finding out her behaviors were considered abusive, continues to do them. My husband promised his dying father that he would take care of his mother. Now, he feels like he has to. He does everything for her, she has always been the laziest person I have ever known. Yet, she complains about how he doesn't do enough, and what he does do, isn't good enough. She does not complain about the other caregivers. I am with him on some of these visits, so I know that he does his best for her. Yet, she constantly criticizes him, and everything he does, she screams at him, cusses at him, hits him occasionally and has recently told him she feels "neglected" by him, that maybe she needs to call someone about that. I got the threat right away, and suggested to her that if her living situation was just that intolerable, that she feel free to move into assisted living. She did a complete turnabout and said to me that she knows he's doing his best. He spends the nights there when he is taking care of her, and last night she got him up out of bed to run the dishwasher! She walked all of the way downstairs to his bedroom in the basement to wake him up and tell him this, when all she had to do was put a pod in, and push a button. Then he said she proceeded to scream at him that he was so useless. He has started setting boundaries with her, that when she starts cussing him out and screaming at him, he will walk out on her. She is making his life hell, and his brother, who is supposedly her executor and makes decisions for her, will do nothing now. Her own sister wants as little to do with her as possible. She is so mean, and selfish and is ruining over lives, causing constant stress for my family. My husband says that he must keep his promise to his dying father, but I feel like it's at the expense of himself, and our family. He already has an impingement on his spinal canal, yet she keeps asking him to do things that she knows will hurt his back. I feel like she is a black widow, she ran her husband into the ground with her laziness, asking him to do everything for her, now she's heading mine that way, and has already started on my youngest son! I am just at my wits end with this woman! Has anyone else ever had to deal with someone so mean who abuses everyone around her and feels entitled to have her family take care of her when she could afford other help or to go into assisted living? Any suggestions on how to get my husband to think of himself first? He's said what he wants doesn't matter, because that's what she tells him! She is a monster in law!