Re: What do I do ?
LOL…sorry but this made me laugh. It’s very ironic that he’s telling your husband to “control” you…yet he claims that his wife is having an affair. Perhaps he should focus on controlling his own wife! 
Well, I have no idea why your friend is choosing to stay in a marriage and keep her children in the same house as this jerk but I guess that’s her choice. You can’t force her to do anything.
I have 1 woman who I consider my best friend. And I see her as my sister. I don’t treat her any differently than I would treat my own blood sisters. You wrote this woman is one of your best friends. What would you do if the woman in question was your sister?
Honestly I think you should continue being a friend to her. Listen to her and give her your advice when she asks for it. Do not allow her husband to control your behavior.