Yesss stop being mad at the girl. He is the one that needs to be kept accountable. He is the one that made a commitment to you. She has no loyalty to you, he does
If your man is flirting/clearly has a crush on me, and I reciprocate not knowing a thing about you because he didn't say he had a girlfriend, don't be mad at me, be mad at YOUR MAN. That's not my fault even a little bit. Your man's a cheater.