He is the only one who can change his situation. The "his-side-of-the-street" that needs cleaning up is he needs to investigate why he continues to tolerate such abuse where more healthy people would not.
Imho, change in either spouse will bring about some kind of change in the relationship. But it can be hard to understand that the change needs to come from oneself and not by trying to force change in the other person.
Your mom isn't a victim, she's integral to the problem.
While I could argue that her mom is indeed a victim, I agree that she's integral to the problem. And she's where change needs to happen if she wants the marriage to be different..
Btw, I could also argue that the step-father is a victim as well as being integral to the problem.
I could probably argue that the kids are victims as well as that at least one of them enables their mother in ways that make it possible for her to better tolerate the situation rather than make changes.
Those tools are only useful for people healthy enough to implement them. Otherwise it's like scraping the barnacles off a sinking boat. In my brother's hands those tools would be useless. And besides, if he ever did try it, my SIL is way too dysfunctional to just go along with it.
Which is why telling others to use certain tools (or to stand up for yourself, leave him, just do this or that) doesn't usually result in them jumping up right away and doing it. Because they are not at the point where they are able.
But that doesn't mean that the tools won't be useful if, first, the person is able to work on developing what is needed in order to be able to use the tools. Imho, too often it's insinuated that someone just doesn't want to do something or likes the hell they are living in. I doubt that is the case most of the time. I was able to use many tools (that were easy for others to pick up right away) after I worked on other skills that were necessary in order to be able to use the tools.
It would lead inevitably to divorce because one healthy spouse does not stay with an unhealthy spouse.
It really depends on what the other spouse chooses along the way. Personal recovery is a process and there are many ripple effects along the way that others have choices in how to handle them as they come. Sometimes it brings about an enjoyable and strong marriage and sometimes it leads to divorce. Imho, personal recovery work isn't done until there is one of those two outcomes.