At this point, if she was not cheating MC I'd consider it as a plot hole.
It wouldn't be a plot hole at all, people lie irrationally all the time when they feel backed into a corner and don't know what to say. She knew she fucked up by keeping it from him, and in the moment probably just blurted out lies in hopes he would calm down and they could work it out. That obviously didn't happen and she got kicked to the curb, then sits in her car crying. Probably reflecting on how dumb she was for keeping it from him, and probably how she could have better handled the situation.
I personally don't like her, and couldn't care less if she even reappears in the story, as I won't be pursuing her anyways. However, I see some really wild and out there takes in concerns to the beginning of the game and her in particular. She is dumb, and honestly if I were interested in her, my trust in her would already be greatly depleted. Though no, just because she's a liar and a stupid one at that, doesn't mean she cheated. It wouldn't be a plot hole whatsoever if it turns out she didn't cheat, and I honestly think she indeed didn't cheat, and that she's just a liar and made a stupid decision in terms of keeping it from MC.
I've also seen the take that her friend (ex-FWB) is the one she actually cheated on, which is also a wild take. We have no context to if they had any rules set up with their relationship, which is the only way it could possibly be considered cheating. Therefore it's a stupid assumption to assume they did and that she just up and left the guy without saying a word after getting a boyfriend to call that cheating. I think it's way more likely they were just friends who had sex sometimes, with no sort of arrangement or rules set in place. Therefore, she has no expectation of exclusivity or commitment. That's how most FWB relationships that I've personally seen are anyways, I rarely have ever seen FWB that are exclusive.