If his unique driving pattern wears that particular tire. What I mean is that if his normal route consists of (for example) frequently taking a tight right hand hairpin turn, it might wear the outside rear tire faster. Depending on the surface camber, braking vs acceleration, etc. it might give a different wear pattern, but if there is a particular place that he drives that for whatever reason puts that one tire position under extra wear, it’s not inconceivable. A different vehicle on the same drive might not have that odd wear pattern.
Another possibility is that for some reason the rear open differential is somehow putting more torque on that one rear wheel, maybe because the other wheel has more friction in the wheel bearing or something, it might just put more torque on the other rear tire.
