Personally, I'm betting that Felix is out and they're doing it 'in the marital bed' so to speak.
We'll probably find out in a couple of days if we can glean the answer from Felix's POV.
EDIT:
BUT WAIT!
A long time ago, when I was still in the discord, I threw out the following idea for a scene :
Zack is fucking Sophie and gets the idea to call Felix over to the frat under the guise of asking him for help with a math question. He'd tell Felix that his door would be unlocked so he could just let himself in.
Felix would agree and head over to the frat. Then, when he opens the door to Zack's room, he'd find him in the middle of fucking some woman doggystyle while she moaned into a pillow.
However, he wouldn't realize that it's Sophie because he'd be seeing everything from behind and Zack's large frame would be blocking most of her body from view. All he'd see would be Zack fucking the shit out of some one.
Caught by surprise, Felix would stammer out a quick apology and quickly leave the room just as Sophie cums all over Zack's cock.
Zack would then come out later and apologize to Felix saying that he'd meant to tell him to come later. Meanwhile, Sophie would be lying naked and semi-conscious on Zack's bed with his cum leaking out of her.
After I shared that idea, one of the devs told me that I was going to like the game because my idea was very similar to one he had already planned.
And I think we're at a good point in the story for it to happen, seeing as how Sophie has acknowledged that she likes the idea of Felix being around while she's cheating on him.
Here's hoping they haven't ditched the idea. *fingers crossed*