Any luck with Currency Exchange??

My sister and her children are expats living in Tivat and I'm trying to help them with an issue.  Recently her and the children went to Poland on vacation and she completely forgot to exchange the polish money back to euros at the airport in Krakaw.

Does anyone have any suggestions on how she can exchange the zloty back to euros.  Have you ever done successfully or know someone who buys Polish currency? She tried at the bank and the money exchange and they won't take it.

Any assistance is greatly appreciated.