Well, they could also issue you a confirmation of the same type, right? You could even include something similar in the content field, or have a special tag, to show that you did it in a coordinated manner. Or do something involving puzzles on your smartphone or QR codes, or include a selfie together, or whatnot. The client can come up with whatever method they want.

I tried to leave it implementation-open, as the idea is that people come up with a bunch of different kinds of confirmation, so that every npub has multiple possibilities for being confirmed, and then evaluating npubs decide which kinds of confirmation they'd accept. You could even use this for 2FA or something.