I get that his relationship with the vice-chief is supposed to be secret. So he couldn't tell his co-worker. But just say you have a girl.
It's ambiguous enough to not implicate, but still keeps you from putting your whole fucking calf in your mouth.
I also get it's Japan. Face is a big deal there. But you wouldn't even be lying if you said you were in a relationship, then deflected prying. Face fully protected, aside from some butthurt single who's now hoping you explode. No big loss.
Honestly, hope this is just bait and he just bails the literal last second, or, if it does go down the confrontation, I hope it goes down the way it would in real life. Feelings of betrayal and a failing plea for forgiveness.
S'like a netorare heroine knowing exactly whats about to happen, doing it anyways, and then still feeling terrible about it.