John, do you already have a tailwheel endorsement or not?
As far as the law is concerned, this is the only thing that makes a difference. In order to get the initial endorsement you need to undergo formal training from a qualified FI (or CRI I suppose), and that time is logged as P/UT (or "dual"), with the FI/CRI logging PIC.
Once you've got a tailwheel endorsement, regardless of the type (as long as it's withing the SEP class), the law doesn't really care anymore, as long as no more than one person logs PIC.
So if you have a group member who is qualified to give "differences training" but who's not an FI, we're not talking about a legal rating or something, but most likely something that the insurance insisted on. In that case he can only "teach" you the differences from one tailwheel aircraft to another, but not give you the initial tailwheel endorsement. And he's not an instructor so if he logs PIC, you can't log P/UT ("dual").
So whether you can log that short dual flight is up to you and your mate: Either he logs PIC and you log nothing, or you log PIC and he logs nothing. And there may be stipulations in the insurance policy that he is to act as PIC during those differences training flights, so you might not have a choice in the matter. Particularly, of course, if only the persons named in the insurance policy are allowed to act as PIC.