I am the pilot in command. You are going to fully prepare for and safely execute this flight (details). I will not intervene unless it is necessary to ensure the operation remains safe and legal.
If you require any assistance during the flight then please ask and I will help.
--------
The above short to the point brief leave no doubt as to who is PIC and what is to be done.
I recomend that you prepare your own version of the plan for the route so that you are not relying on theirs if they have made a mess of it.
What are you trying to do with them?
Aircraft check-out and local are famil or are you doing something a little more eg sharing a return trip to France?
If it is the latter you need to try to establish what their experience is to establish the posibility that you are introducing something they are not already comfortable with which is where instruction would come into it.
Really, I would recomend that you fly with them and if happy then fine but if not then have the weak area(s) covered by an FI before another go. Otherwise you are simply banging your head against the wall.
Have you thought about pairing them with an experienced tourer (who does it properly!!) and allowing some learning by observation / participation.