I think that on entire globe, world wide,on planet Earth, on all continents, there is universal rule for all gun owners:
The one who pulls the trigger bears the responisbility (or cost) for what is done by that action.
Life saving situation in grave and imminent danger does not change that fact.
Moreover, dealing with dangerous animals on entire African continent, was not supposed to be under individual commercial clients tag. It is more likely to be rare coincidence, when it happens.
Moreover, clients safety is PH responsibility. So, if PH is acting under that responsiiblty why the client should pay for that action?
So why a buffalo, who unexpectedly charged, and is killed by PH should be on account of clients bag?
By contract, the client is entitled to his first shot, if it doesnt happen it is not clients fault
I think that this particular situation should be checked under proffesional hunters national organisation code of conduct, or code of ehtics. (I am sure it has been an issue raised before)
One other point, is european continental hunting tradition, the rule that probaly comes from driven hunt practise.
The trophy (or animal) belongs to the hunter who shot the animal first. (it can be finished by someone else, but first shot counts). For birds it is the opposite, the bird and small pray shot with shotgun, belongs to the hunter who brought it down, not to the one who wonded it.
So, once again, who shoots (first) bears the responsivility (and cost) of his action. I dont think that in PH/client relationship is any different