Happy? To be honest, I don't think 'Happy' is in the cards for Felix at this point.
However, I can see him at least having a future that doesn't involve a woman like Sophie. And that by itself would be a huge improvement.
Possibly an (unlikely) ending where he finds out everything and vanishes.
He quits the school without telling anyone except Brooks when he hands in his resignation. When Felix tells him that Zack is the reason he's quitting, Brooks just nods. Since that other school was head hunting him at the conference, Felix already has a place to go where he can start over. He'd secretly make the preparations over the coming weeks, pretending to believe the lies Sophie would continue to tell him. Pretending that their marriage wasn't that lie it had become. But then, when he was ready, he'd pack up all his shit while she was 'out with amy', and just leave without saying anything.
He'd send the divorce papers later, though. Along with his wedding ring.
Maybe he could even place a hidden camera in his house to catch Sophie in that act, and then hand the evidence over to the school board. But I think that might be too out of character for him.
However, the more likely ending, as I've said before, is going to be a perversion of the 'Happy Ending' from the beginning of the story.
- Zack dumps Sophie because he's done with her, but not before getting her pregnant.
- Felix finds out about the cheating.
- Felix still loves her and takes her back, raising the blue-eyed baby as his own.
Pretty sure they are planning a route where Felix ends up with Amy. There was an option early on that was greyed out that seemed to indicate bonding with her. I doubt any ending will keep Sophie, as that's kind of the point of the game, but I bet they'll let Felix waltz off with Amy if he plays his cards right.
Amy also seems obviously uncomfortable with what is going on and feels like Felix deserves better. My guess is she had a crush on him from when she was his student, got used and tossed aside by Zack, and now wants someone sweet and kind.