I think halfbaked boy is right; it's same as required equipment carriage. Local regs override the aircraft nationality, and the requirements are potentially different for each national airspace that you happen to fly through, and that rule is probably broken every few seconds somewhere in Europe because not even Spock could remember all the national details for an arbitrary flight across Europe.
I thought (without checking) that the proposal would not be legal in UK airspace unless the RH was an IR rated instructor.
However, taking even the most pedantic practical view, the [il]legality of it would not crystallise until the moment the LH pilot makes that entry in his logbook, which he's unlikely to be doing until safely back on the ground.