I don't think anyone can understand what your Mom is going through. Unfortunately, you don't really know what the relationship was like between your parents and many people stay with a spouse for the sake of their children..I think it is somewhat noble, but the "experts" say you shouldn't do that. Anyway, being 61 myself, I do know that life finally starts to feel finite and it is not a good feeling to stay in a situation that makes you unhappy.
she is apparently unhappy. I know your Dad will be devastated, but I think most men react better to this sort of thing than women do. He will probably find someone new who will treat him the way he deserves to be treated. Maybe you can even help him with that when things settle down a bit.
I can understand your feelings and I hope you are able to come to terms with them. Good luck and I'm so sorry that you are going through this.