I mean it's clear that their relationship was really toxic with the ex cheating on him and manipulating him. He even says he hasn't loved him for awhile, it just took a good person coming along for him to leave him.
The ex has his own short story it's part of the author's other work. He ends up with a sports journalist if I'm not mistaken but it's been a decade since I read these older works of sensei's
I don’t even remember anything about the story it’s been 5 years LoL
Does the ex not have a story?
Well, if the uke was able to lose feelings for his ex just like that over a "friend" I guess the new lover is going through some tough challenges LoL.