In past experience (leading/instructing people in rock climbing, XC skiing, mountaineering, using construction equipment) I've noticed that when some individuals cling stubbornly, even defensively, to counterproductive methods it tends to be rooted in fear, of either harm or of being embarassed by an awkward mishap.
With that in mind, I've been thinking back years ago to when I was first coached through the body movements of torso rotation. I was fortunate to have an outstanding teacher, a former Canadian national slalom champion and expert sea kayaker. But I can still remember being nervous when first trying it, feeling the boat yaw slightly under the shift in body position on each stroke. Even once I got used to it I still tended to lock my body straight forward when the water got rough. It took the better part of that summer for me to break the habit and trust the boat's stability.
You say your reluctant friend is heavy -- do you think it could be that he gets freaked out by the feel of the boat's subtle position change during that weight shift from winding up with the blade plants? Also, some people with really big bellies don't rotate easily.
I'd be more concerned about his apparent failure to monitor and mitigate his own condition (becoming overheated). The very first lesson I always taught students in any wilderness sport was how to stay aware of their own condition. They needed to recognize the early signs of hypo- and hyper-thermia, dehydration, low blood sugar, etc. and know how to correct them so they would not become a burden to their companions. He was lucky you were attentive enough to spot it.