I agree with heissick. Your H is a grown man and if he wants to move his accounts elsewhere you need to let him do so. As long as he is not wanting you to choose sides in the rift you need to stay out of what's between your dad and your H, even though I know it's very hard to do so because you love both of them.
"I'm sorry you two had such a falling out, and I hope you will think a little longer before you decide to put your retirement account somewhere else. If you are honest with yourself, you're doing this out of spite and it's going to damage relationships further. You are a man and you can do whatever you want, I just had to say something because I love you both and I really want you to be able to get along."
Yes it would be nice if she could get him to hold off a while before making a decision out of spite. If her H can tame his anger for a bit she may be able to reason with him. I feel bad for her that she is in the middle of this though.
I would say this to your husband:
"I'm sorry you two had such a falling out, and I hope you will think a little longer before you decide to put your retirement account somewhere else. If you are honest with yourself, you're doing this out of spite and it's going to damage relationships further. You are a man and you can do whatever you want, I just had to say something because I love you both and I really want you to be able to get along."
You know.