In fairness, I can see getting angry if the dude who fucked her ended up being someone you thought was a friend. If a friend fucked a chick I was dating I would probably go more ballistic on the friend because how the fuck do you fuck your boy over like that?
What a dumb fucking bitch. Both the GF and 'Prudence'. But in the end, if the dude feels he can't trust her to not be a ho then he should just dump her ass and enjoy the peace and quiet.