Damn. Now the GF knows or rather, how long has she known?
I've just looked at the raws andAnyway if anyone is curious here's the rawsi can't read japanese but the gf is very much twisted herself(seems like she's a sadist), i'm not sure she's in on it with the licking girl but she has been spying on them everytime, she seems to like seeing the MC being dominated and suffer at the other girl's hands, i would say it's a mix between enjoying being cucked and at the same time she likes seeing the MC suffer with guilt for cheating on her, there's even a part where she imagines what reaction the MC would have if she caught them on the act, and she gets super turned on by it.
In short, bro needs to get the hell outta there.