The employer contributions are treated as if they are the employee's. Your employee doesn't qualify.

"A person can generally maintain their eligibility to contribute to an HSA after age 65 as long as they are employed, enrolled in an HSA-eligible HDHP, and not enrolled in Medicare or other non-HDHP insurance"

And there is a Medicare Supplement HDHP that also doesn't qualify for HSA, anyway. That's why your taxpayer might pursue MSA, or the State might have an MSA provision.

They need to make a corrective distribution, pay the tax, etc.