In my experience, if he can shoot it well, you can't beat a .30/06 for plains game (excluding Eland).
My load of choice, based on personal experience, would be a 165-168 grain Barnes TSX or TTSX factory load or equivalent, such as the 168 grain Remington "Hog Hammer", which is a 168 grain TSX. The 180 grain TSX or TTSX would be a good choice as well. I would select whichever one groups best in his rifle.
My reasoning is moderate recoil, good accuracy, fairly flat trajectory, deep penetration and the ability to put PG down fairly decisively with a well placed shot. The '06 has been doing just that in Africa for a century. It is tried and proven.
Another consideration is that should you and your ammunition get separated in transit, most PH's have a supply of .30/06 on hand.
I don't think he would go wrong with that setup and a quality rifle of moderate weight, such as a Model 70 featherweight. For scopes, a Swaro Z3 or Zeiss HD5 in 2.5-10 is a good match for that combo.