Most Helpful Guy
If the side kick was lied to originally, well, that's sort of to be expected.
If the side chick happily cheated with him then decided to blow it up, yeah, he's gonna hate her.
If she's thinking that it will trigger him being with her, no, that will not happen.
This is a more common 'guy tries to break up with side chick she threatens him with this'.
I read somewhere it was surprisingly common for people to get blackmailed into continuing affairs by the threat of them being exposed.
Most Helpful Girl
Unfortunately it's a risk you take when you cheat. Things can get extremely messy!! However you said his girlfriend not his wife so I suppose it could be worse for him had he been married.
I also think if this girl knew he was with someone else then she's just as stupid and shouldn't expect him to be with her over the other girl.