Whadya think? Maybe more a statement about their marksmanship rather than the capability of the load?
I put a round ball from a .45 cap and ball rifle through a large Saskatchewan WT doe, that entered through the ribs on one side and exited through the shoulder on the other, a path of about 18 inches. It was not a stout load.
That was at about 50 yards, IIRC. I was really surprised that the ball exited.
How many times through a deer do you need to go?
That said, Kyle, what is your cousin shooting in the way of a rifle now? I'd say take him to the range and see what gives, as far as tolerance for the recoil of whatever load is going to be used.
I cannot recall ever feeling the recoil of the shot when shooting game.

Shooting paper of a bench, now, I have some bad memories of that!
Do some hind legs shooting (standing) and some shooting off sticks or similar, try to shoot the way you would if you were out hunting.
Cheers
Trev