Since the key phrase seems to be "a rifle he/she is comfortable with", then what does it take to get "comfortable" with a rifle?
In my experience it is just getting it sighted in and shooting (and carrying it) enough so that you can aim, mount, and shoot it with field accuracy (based upon the ranges you will most likely shoot at the animal hunted).
You should be able to take the safety off or on without conscious thought. It also means to me, knowing the ballistic limitations of your rifle as far as range.
For my .416 Rem Mag it took me about 2 months and 2 boxes of rounds to be able to shoot it effectively out to 200 yds (that includes sight in rounds). In actual hunting conditions, I never feel recoil or even think about recoil.
If a person doesn't handle moderate to heavy recoil very well, than I would suggest they have no business hunting large DG. JMO.
However, I can certainly see where a guide would rather have a client use a minimally adequate caliber that he has a lot of confidence in, rather than deal with a nervous nelly who is scared of his rifle and can't shoot it for sh*t.
People draw confidence when faced with a potentially dangerous situation from different things. Some from external things like a rifle they have killed a lot of game with, some from religion, and some draw confidence from within.
