I am not familiar with magpie's music but if I was going to go and spend an evening listening to music at a coffee house or other type concert I would first find out something about the artist that was performing. In this case they should not get their money back. Whatever that amount would have been. However if they just came off the street or decided to take a stroll and spotted your event and went in. Then I would probably give them something back. Problem here is that neither of my observations has a defenit answer.

Small claims court!   Wait and see if they file. I bet they don't.
Beer (adrien)