The whole rudder indifference thing is based on not having seen what airplanes can do when you get slow and uncoordinated on a turn-to-final on a windy day. Load the airplane with CG forward (nobody in the back) and tell him we are going to do some uncoordinated stalls today. Don't say spin or incipient spin or he will get scared. Then you show him a power-off clean stall and gently kick the rudder to one side at the break. An incipient spin will occur depending on how long you hold the rudder down. No need to scare the crap out of him, warn him the wing will drop a bit soon. I promise he'll get the idea.
I used to teach for an airplane manufacturer and one of our standard teaching tasks was a cross-controlled demo stall to show what a wing drop looks like. It works because it is the only way to connect rudder input with wing output on an airplane. Otherwise the whole idea remains abstract and in a few days they forget all about it. The cross-controlled stall is particularly valuable because it simulates those windy day, late-turn-to-finals that can kill. Until they understand it they are probably thinking rudder coordination is a pain in the butt with no connection to safety.