I'm not sure it is fair to assume that her exhusband is the cause of the problem between her and her son.
My stbx and I get along very well considering everything he has put me, and our adult children, through over the last few years. I don't speak badly about him in front of our kids and actually don't think I speak badly about him to anyone. I might tell friends something that he has done that drives me nuts but that is about it. Unfortunately for him, his relationship with his children continues to deteriorate more and more as time goes on.
In my opinion, adult children of divorce are the most ignored individuals in this awful world of divorce. Everyone seems to assume that since they are adults and will have their own lives, it doesn't negatively affect them.
Young children are basically forced to go back and forth between mom and dad. They have to learn to adjust. Adult children make their own decisions. Adult children do not adjust as well. Often when adult children start their own lives, deep down they know that they will always have "home" to return to, it's their security blanket as they become adults. When we divorce and take away that "home" it is very difficult on them. My husband left the first time when our daughter was pregnant with her first child. We were both there for the birth and got along well but I know to this day that she resents that her happiness over her pregnancy and birth was at times overshadowed by her grief of the ending of her parents marriage.
I would recommend reading the book The Way They Were, Dealing with Your Parents Divorce After a Lifetime of Marriage by Brooke Lea Foster. This book really helped me to understand my adult children's reactions and how to deal with them. I think this book would help you too. I tried to get my stbx to read it as he was leaving as I hoped it would help him know what to do to help his relationship with the kids but he wouldn't do it.
My husband left me for another woman and I do not know if you left for someone else or not but if so, that could be part of the problem. My kids direct the anger at their dad and his girlfriend almost completely at her. They won't talk to her or have anything to do with her and I would be surprised if they ever do. This is their own personal choice, not because of anything I have said. They both know I would suck it up and do whatever they want. I am sure that other adult children place the blame on the leaving parent. Everyone is different in how they react to trauma and this is traumatic for children, young and old alike.
For example, my stbx is in the hospital facing surgery this week. Second hospitalization in 3 weeks, 10 days the first time. They are very concerned about him but hate having to "make an appointment" to see their dad because neither will go there with OW in the room. In their minds, he has chosen to have her there with him instead of them. In his mind, all they have to do is call and he will have her leave when they come so this is their fault.
They both believe they are right to feel the way they do and feelings aren't easy to change.
I wish I had an answer for you but maybe reading the book I mentioned above and others on this subject would be helpful. Unfortunately, this is not a topic that is discussed or written about very often.