I am no expert, but I believe that you can only operate as a "Co Pilot" on an aircraft that is certificated as a multi crew aeroplane. Even if your passenger holds an FRTL he is still a passenger who just happens to be legally entitled to operate an aeronautical radio. This is why if two pilots fly together (under UK ANO) there can only be (as far as I can see) one P1. Naturally if the other pilot is an instructor, then the instructor may log P1, and the "student" may also log the same hour as P/UT.
This is different in the US. Under FAA regs, even a checkflight with an instructor may be logged as P1, for the time that you are the sole manipulator of the controls. Naturally, I am assuming here that in this instance the person undergoing the check is already a quialified PPL.
Hope that helps.
Cheers