I am totally confused??!! How did you find out that he got married and when did he marry her? I thought his affair was over and he was working toward a reconciliation. Are you sure that they are really married?
I know this must be hard for you. I am so sorry and I know there must be some relief to it, too. You are in my thoughts and prayers. I am sorry for so many questions but I am flabbergasted! There is an old word, I pulled out of the archives but it fits!
Everything happens for a reason, maybe Dad needs to find that it isn't better out there, he needs to realize how good he had it here. Maybe he will find God and that is the most important thing when he finds Him he will know he is supposed to come home.