It seems weird that Koyori's ex would go for that batshit crazy plan. There is virtually nothing suggesting that Koyori would come back to him if she were to break up with Hijiri. At the same time, the plan is a massive career risk. If someone caught wind of them abusing inter-company relations for personal bs, I'm pretty sure it would be firable, if not, especially in Japan, career-ending. A slight possibility of getting back with your ex that you cheated on, to begin with, is not worth getting blackballed.
For now, we still haven't seen what the bitch told Koyori in the toilet, and since Koyori is currently kind of a weaker link in the relationship, that's probably the crux of her plan. Still, I will bet on manga not having a drama tag for the possibility of relatively smooth sailing.
A bit troubling that she still even cares about the relationship from when they were 14 or so since that's a completely different kind of relationship that an adult one to begin with, but with her unearthed feelings kinda coming from the same point in time, I guess it's reasonable to feel teen-level insecurities until they mature. She should probably talk about it with Hijiri, since it eats at her. In general, the level of communication presented is pretty good, so I hope they won't suddenly start bottling up their feelings and issues just days after promising to be blunt with each other.
And Koyori really needs to rain in her drinking. It already went out of control twice in just a few chapters. She's not at an age where you just pour alcohol into yourself with no breaks when you feel slightly troubled, because all that will do is cause more problems. Drink responsibly, kids.