That's true if the back of your hand is facing you. If your palm is facing you, your right hand makes the "L".
The easy way to remember L or R is to make an L shape with your thumb and pointer finger. On the left side the L will be correct, and on the right hand it will be backwards. Maybe that trick can help him.
That's true if the back of your hand is facing you. If your palm is facing you, your right hand makes the "L".