Haven't updated OIC for a long time.  Sounds a possible candidate...

Did a bunch of those 10+ years ago.   Last OIC I did was for state sales tax, which (as a trustee tax) for a long time the state would not accept for OIC, but CA was so rich that it did at one point.

If the amount is significant, look into it.  But then, how can he/she/it/they afford to pay you for filing???