With this being a fantasy world, I'm honestly surprised there's nothing from the fairies' side or any of the other races that could act like a magical truth serum, force it into the bitch, and make her spill the truth cleanly from there.
But okay, I guess I can kind of see where this is going: the bitch acts out out of her own 'pride' seeing the emperor acting lovey-dovey to somebody other than her, accidentally slips something out in her jealous temper tantrum (if she even has an affectionate part in her cold, hard, scaly body for the emperor instead of being with him out of political gain), and gets screwed over.
Well, this is a romcom, after all.