Just put the parking brake on ,guys and RTFQ!
If the POH says don`t, then Dont ! If you do and f%$£ -up your insurance will be invalid ; and this `lad` is about to go off on an early solo sortie.
If, and taken at face value, his instructor cannot demonstrate a manoeuvre adequately/competently,and says such things as `it`ll be alright,if a 150 can do it,this can`, then a change of instructor,and/or club should be considered.
Why does the POH SAY YOU CAN`T? Possibly , because at different CofG`s you may get a pitch-up, or a pitch-down,, you may be overstressing the fin, you may get fin stall,flick,spin, crash, burn, die...!
If he`s trolling, then the message is still the same. Do it in an a/c that is so cleared,and when it has been demonstated adequately........Syc