Would be surprised if anything serious happens.
Every single instance of cheating so far has been coercion/trickery. Here she knows pretty well that the guy is trying to get on her pants. If she goes along with it, then there is no going back, unlike previous cases.
Author has been deliberately skirting the line until now. I don't think this is when they would cross it. I'd definitely expect more misunderstandings, like him seeing her come out of fuckboy's house, assume the worst, and then have Fumi make her move.