LLC Company in GA has closed. We are being sued. What are our rights?

Our Plumbing LLC Company closed. A previous customer wants a refund. The client waited over 18 months to call for the plumbing to be completed. (they ran out of money, as most people do.) We closed the business before they called for completion. We tried to offer them a portion of the refund they were originally asking for. They cashed 3 checks for the overpayment, then decided not to take the offer. They want the full amount refunded. They are threating to take us to court. What should we do? Thank you so much for your help!