I mean, her boyfriend tried to close the distance many times in just a few chapters we've seen...
She does come off a bit as "I might not really like you that much" or something. Moving on doesn't sound so unrealistic if you ask me.
Exactly, if anyone is "in the wrong here", it's her.
Her opening up, befriending and acting like she is to her coworker/neighbour is already enough of a reason for anyone to think she is or is going to cheat.
Osamu's scene is pretty much identical to her scene on the guy's apartment.
To me, this reads like she is a teenager, and the other characters are all adults smh.