Business pays Medicare / Supplement premium for Sub S Owner

Plan uses 415 safe harbor comp.  The Owner of a Sub S Corp is now on Medicare.  The business pays the Medicare and Supplement premiums on behalf of the Sub S Owner.  Would this premium be added to the Sub S Owner's compensation for plan purposes?

Yes, These are taxable fringe benefits. See Form 1120S, Line 7 instructions. They are included as Officer compensation and their W-2 Box 1 wages, but not Boxes 3&5.

See Form 1040 Instructions for the Self-employed health insurance deduction. All Medicare premiums are deductible as long as the individual is not eligible for employer subsidized group health insurance coverage of themselves, their spouse, dependent's or child < age 27. While Medicare supplement plans are not explicitly stated, I would assume those are also deductible.