Credit card purchase issue

We sell photo booth machines in USA and we sold one to a client in CA in 2014 (we are in NY) and it was delivered in January 2015 - cost was $6500 and they paid with Discover Card; 18 months later nearly two years after the purchase we get a call from Discover and they have the purchaser on the line -- did not tell us and the purchaser s claiming the printer broke down and we did not fix it for her and she says other parts don't work too! - She is trying to get a full refund nearly two years later - our return policy is 10 days - anyway - we felt it very odd that discover would have a person on the phone and call us - the purchaser then went onto lie - we have never denied anyone repair or replacement under warranty but her machine is now out of warranty as warranty is one year on parts; Can Discover take back the full amount?...surely not - the guy from discover said he would call back Wednesday and would not give us a email address so we could send evidence to him - please help