OK, when I was in the closet I was a raging asshole. Why, I was pissed the fuck off, I was gay, I knew it, and I didn't want to be gay. I was pissed of at the world because I didn't think it was fair that I got no say in the matter.
The fact that I was in a closet I didn't choose, didn't make me a nice person in an unfortunate situation, it just meant that I was a raging asshole in the closet. There is no excuse for the damage I did while being a raging asshole in the closet. It's why I don't have any respect for gay men who take other people down with them. I don't respect the person I was when in that situation. I knew I was wrong, I didn't care.
We as gay men tend to have a default sympathy for the gay guy; and that's a good thing if he's being fired or bashed, but it isn't a good thing when it leads us to ignore serious behavioral problems on the part of the gay guy.
At this point in this guy's life what he's doing is incredibly destructive to all involved - even himself, it needs to be described in the starkest terms. He's a user who's going to crush this woman. Sooner or later, in one way or another. There isn't even any way to avoid it, all you can do is wait for the crash, and this isn't going to be getting drunk and picking fights - he's going to crush her in a way that's incredibly cruel. All of her hopes and dreams, her happily ever after, her domestic security, will be exposed as empty lies he's told her. Before we start justifying and minimizing what he's doing, let's ask what we think that's gonna do to her?
This guy followed another guy home and made a move with the intention of committing adultery; after telling said guy that he doesn't love his wife, that he only married her as a beard. I'm sure that felt very cathartic for him, but what position did it put the OP in, what does that mean for his wife?
People can change, if they want to, and it's certainly possible that somewhere down the line he'll be a different guy, but he's going to have to clean this mess up first. The only way he's ever going to change is if he comes to terms with himself, and deals with the damage he's done. Until then he will not be a nice person in an unfortunate situation. Nice people care about what they do to others.
If the OP thinks he can help without getting entangled, more power to him, but ultimately this guy made the decisions that created this mess, and he's going to have to make the decision to get himself out.