Tbh, they don't seem compatible at all.
• She doesn't like home dates. While it seems like it's his go to... because of course his house is a good date spot.
• She probably doesn't like that they alway eat barbecue. (The price is besides the point. She got tired of it, maybe to the point of getting used to the high quality... could have at least told him that though.)
• I have not gotten to it yet, but maybe she got used to the Twilight brand that he frequents too (and thinks it's a normal shop), or she just has a go to brand that's not Twilight and she buys her own clothes.
Her cheating is really dumb tho. Like, why?
Maybe the guy was courting her incessanty, and when she finally gave in she also jumped the gun and said yes to the guy's proposal. Her mind clouded by the honeymoon phase of relenting to who she thinks is a better romantic partner and the promise of stable finances? Feeling of being pursued vs the MC who stopped pursuing and she gets stressed with because his job is not safe and is unstable, in her head the choice is obvious... but also why does she not know he's high ranking, don't they talk? Fucking situations that shouldn't happen if you just talk.
Actually she didn't seem like she was a total raging bitch, just normal bitch for a girl specifically trying to be bitchy because of course she's doing that, she wants to cut ties and she's preempting aggression. Very toxic but some people do that when they think the other person would do something like get angry or beg. Or she had some reservations about it, but she likes the other guy much more now and she's riling herself up to justify it to her own self.
Like going "I'm totally pisses!" inside her head even tho she's actually not.
Maybe they've actually drifted apart in some ways in recent time so she's particularly easy to to sway.
This is just me trying to figure out why and how this situation could reasonably happen without immediately marking a character as brain dead stupid, evil or some such.
This could just be a flaw in the writing too and she isn't portrayed how the author imagines it, maybe she really is a bitch and that dumb.