I agree that this should be throughly discussed by the client and PH before the hunt. I also see too many TV hunts where I think the PH shoots too quickly after the client's shot. It's a judgement call by the PH. I like the way 458express operates. Any dangerous game animal coming at you at 15 yds must be stopped immediately.
I also think part of the reason PH's have their clients check the zero on their rifles is so the PH can see how experienced a client is with his rifle.
I've hunted with about 10 different PH's on plains game hunts, and the only time any of them carried a rifle was the 5th day that we spent trying to find a wounded waterbuck. In that case it wasn't a matter of a wounded dangerous animal out there, but of a lost wounded animal that the hunter would have to pay for.
When I hunted my cape buffalo in Zimbabwe, my PH carried a .458 Win, but the only time he loaded it was one day when we were stalking a buffalo herd in thick mopane and we saw lion tracks on top of the buffalo tracks. We were close enough to the buffalo that we could hear them grunting and walking through the brush.