What have you been smoking?! :) Maya didn't cheat. It's obvious Maya & Stepan's relationship changed 3 or so months ago. They minimize any conversation with each other, they are rarely in the same room together, they don't leave the house together, they don't cook together (or for each other) and they haven't had relations in 3 months. They are in a "roommate" relationship for no other reason then economics.
And...now Maya (without Stepan's help) is building a partition. I can't imagine that in place in what is already a small apartment. But just the fact that he isn't helping her with this project more so tells me they are roommates.
Postscript: Looks like the partition is being used for one of Maya's personal projects: Molding clay into a giant dildo. :-)