How was it reported by the employer?

Assuming this payment was made in 2019, I'd suppose it should have been reported in Box 3 of 1099-MISC and that would be reported as Miscellaneous Income on the 2019 return.

Regardless of how it was reported, it goes on Line 8 of Schedule 1.  If there was income tax withheld, though, I would be tempted to use Form 4852 and call it a missing 1099-R.  For withheld FICA taxes, that's a matter between the taxpayer and the employer.  

Hi

It was reported on his W2.

The W2 was in the name of the deceased spouse.