This story is something else
As a couple headed for a life together they are history. As you point out she made the choices that led to the ending of their relationship. Their relationship might have been salvageable if he hadn't felt compelled to watch the video over and over again - he said he didn't know why he did that, but I don't think he's being honest with himself, and I think that's also why he is now repelled by her. Psychiatry, psychology and counseling is filled with people who are woke leftists, so the chance of therapy being helpful is pretty slim. I feel sorry for both of them.
For starters, this guy is trying to make internet strangers responsible for how he feels about what his fiancée did. He’ll only come to terms with what he does if he owns his decision, regardless of how other people feel about it. If he’s disgusted by her and wants to break it off, he should act as if because he’ll never be at peace with staying with her, no matter how much strangers affirm his decision. And if he wants to stay with her, he should stay, forget other people’s opinions. Same deal.
He should stop feeling ashamed of his feelings, so he can truly figure out what he wants.
“I believe her, because she was always honest with me.” No, at this point all of her energy is focused on preserving the relationship-reputation management. She’s owning up to what she could never deny. More than likely, there’s a lot of stuff she’s still hiding. That could also be contributing to her refusal to escalate the issue. As sad as she feels right now, it’s contained to this single incident and no one is talking to the other people involved in the video.
Finally, I’m guessing this girl did some things she’s never done with her guy, and that’s what really gets him. These guys in the video use her, don’t care, and get her best sexually, and fiancée gets the lukewarm leftovers.
My advice is to stop trying to talk himself into falling in love with her when he doesn’t really feel that way. It’s only going to make him miserable.
This is a nightmare scenario for anyone; obviously there are no winners here. Luckily this wasn't like a drunk driving accident where people died, but the damage to a committed, monogamous relationship would almost certainly be fatal. Anyone who loves their spouse would feel jealous and humiliated about a sex tape of their fiancee being shown to family and friends. This guy needs to move on because he has seared this into his brain and he won't be able to get past this. It's tragic, but some mistakes we never stop paying for...