Like I said, the insurance company will do their best to get out of paying for a big claim, including watching how you use a courtesy vehicle, if you are the person using the courtesy vehicle all the time, they have reason to assume that you were the main driver fo the other car.
If you are insured as a named driver, they expect the main driver to do the lion share of the driving on the car. If the younger less experienced driver is doing and equal / greater / all share of teh driving, the insurance company would re-calculate the premium based on the increased risk.
It's up to you, but I wouldn't do it. I've got enough points as it is, without having the hassle of having to battle to get an insurance company to pay out when I need it most.
Lying about who the main driver is, is exactly the same as not declaring mods, saying you have 10 years NCB when you have none etc etc, just not worth the risk IMO.