I've heard of an IRA being funded by an employer . . . through paycheck deductions.  The taxpayer should have been claiming IRA contributions each year on their 1040 which would either result in a tax deduction or tax basis in the IRA (reported and accumulated on Form 8606 each year).  My first step would be to go try to unravel that mess.

However, I've never heard of a SEP-IRA with tax basis (but I've not actually seen a SAR-SEP, they went extinct before I got into this business, so maybe it has to do with that?)

For purposes of determining the amount of tax basis recovered tax-free when taking an IRA distribution, all of your IRAs are considered to be one big IRA.  This includes SEP-IRAs and SIMPLE-IRAs.  See Form 8606 and instructions.

Rick