Pretty sure the girl broke up with him first. And anyway, it seemed like they were around each other a lot longer before the dude went away to the military, and he kept sending MC letters. MC seemed to start thinking only about the other guy, hence the break up. Judging by their reactions, it seems like the girl have had enough and broke up with him, and later regretted it, since she still loved him.
even if he didn't end the relationship he still fell for the guy and she only broke up with him because he didnt love her (I still think he was being unfaithful for continuing to meet up with someone who said they liked him while in a relationship) but ur right I probably missed how much time passed as I thought they only met up like a few times
nasty ass relationship
he smoked with him like five times and then breaks up with his girlfriend that he was thinking of marrying???