I had this experience many years ago, and it was devastating to me. I have always been a daddy's girl, and we commuted together for more than a decade, so I got to know him more as a person, and less as the perfect human being. My dad died 9 years ago now, and there's not a day that goes by when I don't miss him. I never told my mom or any of my siblings about the long-term affair he had. I didn't want to hurt them, and I didn't want them to remember him for the worst thing he did, instead of all the millions of amazing things he did. Parents screw up just like everybody else. If your parent has done something that harmed another person, you should encourage him to make amends, but don't get in the middle of it. The dad you've always known is still in there - he just made a big mistake and needs to own up to that.
Many Vietnam Veterans that I know, who are in their 60ís, have spent all the rest of their life being haunted by the worst thing to ever happen to them. That is so sad to me. After your dad takes responsibility for his actions, and you've gotten to a place where you feel you can trust him again, go back to being his daughter, and try to forgive him for the one horrible thing he did.
I hope this helps. My very best wishes to you and your family,