If the passenger is over the age of 18, they are liable should they be caught not wearing a seatbelt, not the driver. In that case, it would not matter. Under the age of 18, I would be asking to see it, as you are liable. Passengers are always entitled to claim for injuries through your insurance if you have an accident. Should your passenger be advising they are exempt on medical grounds and they are not, and you do have an accident, then more fool them. The insurer would be arguing "contributory negligence" which could see any award for compensation reduced by as much as 80%. Make your passenger who is advising they are exempt aware of this. It may well change their behaviour.