I also have shoulder problems, and I can tell you what my next bow will probably be - Merlin with eccentric wheels. Others that would fit the bill would be Hoyt Protech with eccentric, or Reflex Carribou. Or, I would suggest a longbow/recurve.
The most important thing for your in-law to remember is to not over-do his shooting sessions.
Good advice. My sessions on the compound involve only about 15 arrows twice a week. Quality much more important then Quantity especially in these situations. but, be consistant and try not to skp any sessions.