no scientific proof here,but...
1)his form will eventually cause an injury and you don't want to have any part in that.
2)he is wiring the nervous pathways in an inefficient manner (not full ROM, possibly "overfiring" small movers before prime movers.)
3)current form protocols have come after years of observation/practice/injuries, evolution always wins, we'll talk about it when he's ninety and in pain.
Ask him: why is he afraid of doing the prescribed exercise?( after all proper form is the prescribed exercise, right?)
For reference my crappy form squat weight is higher than my perfect form squat weight, but my athletic performance has benefited from lower weight perfect form squats, in that it is firing my dormant hamstrings....which I know in the future will lead to much higher weight perfect form squats.
I guess what I'm saying is you could try the angle of addressing the weakness that his poor form is trying to cover up.