However, if the MOT fail is something critical and the police pull you over and spot it - valid MOT or not, they can still impound your car.
Only if he is driving about as normal without a MOT. If it fails, he gets it repaired and then he books it in for re-test and get's pulled on the way there and proves its booked in for a MOT and he was going straight there he'll be alright.
Unless he has shopping in back seat or is like 50miles in wrong direction from the garage.