Piper.Classique and bose-x,
I am asking this as an open and genuine question not trying to have a go or prove a point.
I understand of course that as the PIC and owner of an aircraft, the last thing you would want is a passenger (let's assume a licence holding and current one, but still a passenger) jumping on the controls, especially if you knew what you were doing.
But now lets ask the reversed question as before, but please, less about what is legal and what articles prohibit it.
If you were a passenger with your friend flying in his aeroplane, and you noticed a situation developing (for example, he's getting low on the approach and just keeps pulling stick back without doing anything to correct the lowering airspeed), what would you do?
Would you do as people have already mentioned here and call out the speed to make him realise?
If you did this, maybe two or three times and it seemed like he was still not correcting (perhaps he is overloaded for some reason), would you really sit and let the aircraft stall with you in it when you are perfectly capable of saving the situation?
Again, I am not trying to make this an argument, just a discussion as from what I read, you are completely against this happening whatever the situation (aside from incapacitation), but perhaps I have just mis-understood.