The ANO is quite clear on this. The pilot can occupy any seat provided he can reach all the controls.
In the PA28 you cannot reach the fuel selector from the right hand seat. Having another pilot there - fine. I often fly the '28 I have a share in from the right hand seat if my partner (flying LHS) isn't happy about a short field approach say. However - that means there is still a pilot in the LHS who can call speeds and make any other necessary comments from reading the instruments. The pilot RHS / Passenger not qualified LHS is sheer stupidity - instructor qualified is a different matter.
If you let this person do this - you are heading for a repair bill!
CM