Doesn't the agent need to book the performer in order to recieve his or hers fee? In other words, wouldn't it have to be something like "I'm Such and Such representing So and SO"?? Otherwise, you were never informed of any agent/booker, thusly I think their fee is killed.
Plus, when you met this perfomer, was she already signed-up with this agent? Can you be held responsible for parties that fail to represent themselves properly?
I don't think you should have to pay one red fucking penny unless either the agent contacted you prior to meeting this performer regarding her availability for work or the performer informed you of her obligation/representation by her agent.
By the way, doesn't the agency cut come out of her fee?