"He paid $15K in self-employment tax in 2023"

What tax savings would this be? He still owes FICA, but now he will owe both Employer and Employee share. Plus, he didn't do payroll, and he would need to do payroll as an S Corp. Then, of course, there are the other things he is late on, so penalties and interest tend to apply.

As a Schedule C filer, he gets half of the SE tax as a business deduction, but it happens as an adjusted calculation. As an S Corp, he gets half of the FICA as a business deduction.

Where's that savings coming from?