Forget he's a plumber.  Say he's an actor.  He gets paid to be the understudy for a leading man who always stays healthy.  He never sets foot on stage.  You think that's not self-employment, anyway?  (Well, maybe he could be paid on a W-2.)

If a customer's apartment flooded, my guess is that he would have been there to prevent a claim being filed against his license and his bond.  Until Congress passes a law that the first $10,200 paid for not working is excluded from income, I would tell him that's what he gets for telling his brother-in-law his SSN.