Well he is dealing with betrayal and lost trust among many other emotions.
That is his Dad.
Who is no longer looked upon very well.
His Dad already denied it. Your Husband already confronted him.
Mostly, if he can just 'get' that his Dad is a lying cheat. Even if the Dad admits that, what is that going to change? Nothing. His Dad cheated. The deed has been done or is still happening.
No one knows, how his Dad and Mom are, in their relationship.
I really, don't think confronting them all, is going to heal things or even make things better. It may even make things worse.
Your Husband does not trust his Dad. That is valid.
Your Husband could have a man to man talk, with is Dad, telling him that he is fed up with the lies and does not want to expose his dear daughter... to him. Because his actions and behavior/lifestyle, is not, healthy... for all.
Your Husband can see a Counselor. Of his choosing. It has to be of HIS choosing... because this is his issue and he has to own it. Which I am sure he does.
But having bitterness... is going to affect his own family and daughter.
So, perhaps if you explain that to him... he will understand.
Don't 'attack' him. But merely in a loving way, sympathize with him and inject that, bitterness long held, will affect you/his daughter/ himself... and your family.
Ask your Husband, to heal and help make your family 'healthy.' Again.
Your Husband is suffering a "loss." It is a form of 'grieving.' So understand that.
And anger/bitterness... is one step of grieving.
There are community "grief support groups" in many States.
Maybe he can attend one of those.
When my Dad died, my Mom attended a grief support group. In that group, she said there were MANY people experiencing MANY types of "Grief." Divorce, death, estrangement, etc.
ALL of these, are forms of 'grieving.'
Your Husband, is going through a lot.
all the best,
Susan