A few things come to mind hope something here helps.
Have you tried Refocusing the Scope to His Eyes and Glasses??
You would be surprized how many people don't know you can/should do this or overlook it when it comes to scopes.
Does he have Cataracts (SP)? These alone can cause vesion trouble with optics,esp depending on the light level ie early or late in the day.My own uncle had this trouble for a few years until he got his taken off.
Could also be his Frames or Bi focals if he has them.I have had trouble a time or two with my frames blocking my sight picture
one reason I love the Redfield "Widefield".My Uncle has had trouble with his now Trifocals,luckely he can see at distance better than I can btw,and doesn't wear his while hunting.
Just my opinion but I don't think going to 50mm OBJ will fix his problem,the scope he already has is pretty darn good/clear and bright scope.Upgradeing to better a scope with better glass/lens coatings might be of some help he would just have to go and look through them to see.
Good luck to you and your uncle.