I have first hand experience with shoulder problems(bad A.C. joint) and archery, man it is not fun. Light draw ANYTHING feels good. I' ve had problems since summer 2000: cortisone shots, therapy, drugs, and finally surgery in Jan. 2002. Still not fixed up. I can shoot 55-60lbs. on my new Razortec, but not for a long time. 20-25 arrow practice sessions and my shoulder tells me " no more." .....so I just quit while I' m ahead, and start again the next day. This system works for me personally, took 12 deer since then, didn' t lose/miss any. The most important thing for your in-law to remember is to not over-do his shooting sessions. Shoot what feels good, and only practice when he feels good. If it hurts when he starts, it will be worse the next day. And by all means, if it starts to hurt midway through a practice shoot, stop right there. And yes, like the previous guys wrote, a wheel bow or soft draw cam is better.
Mikey