They are dealing with this (even though on the surface it may not seem so) Everyone handles things differently. They may still be in shock even after a year.
This is a dramatic change that the two of them are getting used to. If there is to be any sort of acceptance on their part, it does take time (there is really no time frame. Each individual processes the information as best they could)
You've already brought up the suggestion of therapy to them. They are aware that it is still an option. There may be situations that a person can resolve alone (depending on their coping skills and personality style) Some people resort to talking to others (not counselors), using bibliotherapy, engaging in soul searching to find the meaning behind the things that had taken place.
Forgiveness is going to be a big part for them. They do not have to approve of his behavior or make an excuse for it. If they view their dad as someone with his own faults and desire for happiness (whatever makes him happy) then, they may withdraw their judgment and try to move on.
You have done well to not beat up their dad in any conversations (the issue is between you and him and between his consciousness/choices that he had made for himself)
It may appear selfish what he is doing but if one remains objective, then he is doing what he wants to do in search of something he thinks he needs.
Forgiveness Is a Choice: A Step-By-Step Process for Resolving Anger and Restoring Hope (Apa Lifetools) by Robert D. Enright (Hardcover - May 2001)