If the other person was a PPL holder and was down as PIC, bose-x would only be a passenger in the eyes of the law.
Any such 'passenger' taking control from the Commander is very likely to be viewed as carrying out an act of 'unlawful intereference' - if it all turned to rats, the insurance company would have a field day.
The ONLY safe option is for any check-out (except, perhaps, amongst members of private owner syndicate) to be conducted by a FI or CRI(SPA) who will act as PIC.