Tiger, the question posed by Genghis was about where was the relevant ruling on this topic.
Despite 5 pages of bad tempered unsubstantiated nonsense nobody (myself included) has come up with the solution. Is it legal or not?
Whether it is sensible is totally dependant on the circumstance. A newly minted PPL letting a mate "have a go" is concerning, but it isn't necessarily going to cause an accident.
Law is generally written for the lowest common denominator and in this case I suspect that the powers that be would prefer only qualified people at the controls, but any law like this is basically unenforceable, so it makes it pretty irrelevant.
If people use their common sense then there is no issue to argue about.