I don't see what was wrong with the dude to deserve that though. It's not like he cheated on her. He properly broke up first. No need to be salty just because he managed to do what you failed to make it happen to yourself lol.
well, first, it is comedy effect, they probably just turn all black like Tom and Jerry, probably.
Second, it is too over the top, yes. But the overall intend is, it was the twist of the usual NTR scenario trope, turn the NTR boy to good guy, then "boyfriend" is the scum one. of course irl you need to judge different. but it is a writing piece, The whole she was dump so she go full slut arc, abandoned the "good girl" arc is very over the top it self and there are not mean to take serious. the author intend to portray boyfriend bad , MC realized that and moving on and find something else. He dump her because she is too "good girl" which mean he can't get sex from her after what seem like the next day, with the
kouhai in same club as her. that detail is there for a reason. This is author say they ain't just become a thing only after he broke up with MC. again, don't need to take this too serious as irl.