Considering two of your points. To fly MPA instead of SPA will in future require the PIC to hold a multi crew rating, if I am reading the new EASA legislation correctly. The intended PIC for this situation does not and is unlikely to ever hold a multi pilot rating as he has no commercial aspirations.
With regard to the medical situation. He took a class one medical but has since had a restriction placed on his medical (OML) - two crew only. He was though offered a class 2 medical instead, which would have no OML attached as they cannot be pegged to a class 2. That should allow him to fly SPA and achieve his ambition to complete the required training and skills test.