Again, from the legal perspective, the following is an extract from the UK Air Navigation Orders 2009:
"Requirement for appropriate licence to act as member of flight crew of aircraft registered in United Kingdom
50.—(1) Subject to the exceptions set out in articles 51 to 60, a person must not act as a member of the flight crew of an aircraft registered in the United Kingdom without holding an appropriate licence granted or rendered valid under this Order.
(2) An appropriate licence for the purposes of this Part means a licence which entitles the holder to perform the functions being undertaken in relation to the aircraft concerned on the particular flight."
Item (2) appears to indicate that in the UK, if a passenger is manipulating the controls, other than simply following the PIC through on the controls, that they are required to be appropriately licensed (unless the PIC is a qualified flight instructor).
That said, from a practical and safety perspective, if I am flying with a non-licensed person, I am going to be giving them some basic instruction in order to give them the ghost of a chance to survive in the event that I become incapacitated. I would feel irresponsible towards my passenger if I did not offer them this safety initiative, regardless of what the Regulations say.
As for letting a mate fly, Rule #1 is "Don't be found out".