I think the point here is not whether they will be pilot in command for that particular flight but whether they are
entitled to fly as pilot in command. I would say that a pilot, holding a student pilots licence, who has completed a solo in the type is, by definition, entitled to act as pilot in command, whether they do on that flight or not is irrelevant.
All the above having been said, if someone is going to do something stupid when you are not there then there is little that you can do to stop them, presuming that the brief they had was explicit.
9mm behind the left ear usually stops this becoming a repetitive error!
