I like your friend already. I’ve caught my children helmetless a couple of times, and each time I’ve taken their bicycles away for two weeks. It seems to have done the trick.
I was recently asked to do a talk on car/bicycle interaction safety by my daughters drive ed instructor. At the end, I brought out a friends helmet after his crash. It was totally shredded from the impact. While he was seriously and non-permanently injured, his is alive only because of the helmet.