They probably got an IRS letter a few years ago telling them they had a credit and to file a return by XX date in order to not lose it.....2014 was a looong time ago, they may be outta luck.

They'll get credit for the estimated tax payment on the 2014 tax return but they won't get a refund.

My experience with the IRS is that they are honest... If an overpayment exists that the client was unaware of, then eventually they would either get a letter informing them of this or they would send the overpayment to them... Just my opinion...