File 2019 with all the income that was reported - W-2 and Social Security. Client should be taxed on the W-2, since it was received in 2019. They can "get even" in 2020. If the client had run down to the office on 12/27/2019 and paid it back, THEN they would not be getting a W-2. I suspect they did not know that. 

File 2020 file for a credit or deduction under the Claim of Right. Look at REPAYMENT in Pub 525  https://www.irs.gov/pub/irs-pdf/p525.pdf