The inference was there, and I’m very sorry that she’s doing that to him.
I follow a blog that’s basically all about getting your own life back after infidelity (it’s not about reconciliation or saving the marriage; in the vast majority of cases it turns out to be a waste of time). I’ll post the link because there may be some useful resources in it for you, and possibly your dad; there are several years worth of posts now, but you can send the blog owner a message. Most posts are responses to letters she publishes, and she has a book as well.
ChumpLady.com - Leave a cheater, gain a life!
I hope you can help your dad, it sounds like he’s really hurting and confused right now, understably.