I don't know. This is really tough! I know it'd be a hard situation. I for one couldn't leave my partner if his dad was abusive & he chose to keep him in his life. That kind of decision is strictly one for the person having to deal with it, not the spouse. A spouse can distance themselves if need be. I'd be a lot more worried about the damage this man has done to the people he was supposed to love than I would anything.
You can't make
or be angry with someone for making a decision like this. You know, it's hard when it's your family. They can treat you like shit and you'll still love them. Abuse though? I personally think it's unhealthy to keep an abusive person in your life, but I can't say what should or should not be done because I don't know the situation personally, and I have not personally had to walk in those shoes. I don't even know what I would do.
I will say one thing: if this woman has children with her spouse, then her biggest concern should not be that her spouse chooses to keep being around his abusive father, but it should be for her own self & children. I certainly, absolutely can say that I know I wouldn't allow my children around this spouse's abusive father, whether he chooses to stay in contact with his father or not, the spouse & kids do not have to.
Say it was me in this situation. I'd say, okay, my partner is a grown man. I'll give him my own opinion and share my concerns with him over keeping a relationship with his abusive father, and then the rest is up to him. But I will make it 100% clear that I have no obligation to tolerate or be around this abusive father and I absolutely refuse for my children to be in any contact with him. That is how I'd go about it. That is what I would make clear. He could agree to those terms or get the hell out. Lol. There's nothing wrong with her wanting to keep her own safety for her & her kids (if she happened to have them, or ever plans in the future to). And surely if he'd been abused by his father he would be gladly agree to his wife & kids not being around his father just because he still wants to.
I'd just be careful, give my advice & opinion as an outsider and then let him do what he feels is right. I can understand that he loves his parents. I would just ask that he be careful and not expect me or my kids to be around an abusive person.