"and their health insurance premium is $28,000. What would be their self-employed health insurance deduction?"

Not the $28,000, unless your taxpayer has other earned income sources, if this was not included on the W2.

Read the instructions for Schedule 1 Line 17, here:

https://www.irs.gov/pub/irs-pdf/i1040gi.pdf

Edit: I found a nice explanation for spousal attribution and for the W2 reporting and for earnings:

https://www.coredocuments.com/health-insurance-s-corporation-2-shareholders