I'm assuming this discussion is about an airplane (or other non-glider) pilot who has chosen not to renew a medical for some reason, and is now also more than 24 months since the last flight review.

Best I can tell, there is no requirement that a person receiving a flight review must be legal to fly as PIC in the airplane (have a current medical). So as long as the instructor is current for carrying a passenger, and willing to sign off this pilot, then a flight review can be done in an airplane prior to soloing the glider.

Am I missing something?

Tom