I'd be pissed and I'd expect my mother to deal with it her way. I wouldn't get involved past that. It's her marriage, not mine. I'd still love my father and want my father in my life, but I would be angry he hurt my mom, but in time I feel I'd get over it. People make mistakes and they deserve a chance to atone somehow.
As for "letting his party down", fuck politics, this is a family matter.....unless they determine the Mann Act was violated, then it's a federal matter and that makes him a stupid douchebag instead just a plain old douchebag.
Again, they need to legalize prostitution. It's a victimless crime. The only time I would demand laws is when it involves children or women forced into white slavery. Other than that, fuck away. You're not hurting me.