Does the mate qualify as PIC capable or does the differences sign off lapse?
To comply with the ORS exemption the Passenger needs to be qualified to act as PIC. No mention of being capable, competent or current - but then why would it as it neither gives, nor implies, any role to the 'qualified' passenger.
To be qualified to be PIC in a Tail Dragger does require differences training and I do seem to remember that, if not utilised, the 'validity' of that training lapsed - but this may have been a recommendation, rather than mandatory.
Would it be a good idea?
NO
the pilot uses a technique that the Pax pilot thinks is dangerous and decides to take over
Then the PIC should punch them in the nose (if possible), Squawk 7005 (if Transponder Equipped), land as soon as possible and have their Passenger arrested for Unlawfully Interfering with an Aircraft in Flight.
It does not matter whether a Passenger is a child, a valid PPL Holder, a valid ATPL Holder or an Admiral from Star Fleet Academy they are still a Passenger and have absolutely ZERO authority in flight.
This ORS does not change that. It just means that, besides the PIC, one other qualified pilot may elect to sit in the aircraft whilst a 'not very current' pilot flies the aeroplane.