If there was an agreement that she would repay you for any charges she made, that agreement *is* enforceable--basically, it was a loan agreement (she borrows money by charging on your card; she would then need to repay the loan). If there was no agreement that she repay, but instead you were letting her use the card as a gift, then she does not need to repay it now--you cannot change the terms after the fact. So the issue will depend on what exactly was said and agreed to when you gave her the card. If the agreement were oral or verbal, it is still enforceable legally--though as a practical matter, it can be very difficult to prove the terms and even existence of an oral agreement, if the other party to the agreement disagrees with what you say.