Her behavior was inconsiderate and slightly shady. She might be sorry that she may lose you over this situation, but she should have thought about that possibility while letting her ex buy her drinks. And she should have been thinking of you, not her ex. Also, under the circumstances, going back to his place was a very poor response to your concern. I would dump her in heartbeat, but then I've been cheated on by my last two girlfriends.
Good decisions come from experience. Experience comes from bad decisions.