I'm not a PH, but I would say the choice of rifles should start early on in the conversation when safari planning. And if a client has a specific idea or "dream" it should be addressed and a plan for preparation should be constructed. Many of us do this by nature because we have been doing it since childhood, but I'm sure there are guys out there that take up big game hunting like some guys take up golf....they figure if they show up with the right kind of gear they can fake it until they make it. Just not so when hunting critters that can fight back.
My last safari was a case in point. I had the idea that I wanted to shoot 2 buffalo with express sights at close range. I stated this up front so it was on the table. My PH and I had a good discussion about my experience level and what I planned to do to be ready. It was comforting to me that we had this level of communication, we also talked about fitness level which I was more concerned about him than myself in that regard. I had initially planned to use a DR but half way through what I term the "8-12 week" training period leading up to the hunt I felt I was not proficient enough with the double, so I switched gears and went with a GMA 500 Jeff bolt rifle. I proceeded to have 3 range sessions a week until I had gotten at least 200 rounds through it (off sticks) and had my hand loads tweaked to perfection. This is what I would consider a minimum to get ready for a safari.
This is when it gets interesting and an example of the f*^(*_((* up stuff that can happen. Of course my bag with the ammo in it got lost before it reached JoBurg. Big issue, where to get 500 J ammo, that's all I cared about. Safari Outfitters -Pretoria, kudos brothers for taking care of me and having what I needed. Literally had to re-buy everything that was in that bag including clothes and binos. Bummer having to spend 2 extra days delaying our time to get to camp in Zim but so be it. Now, I was not only trying to accomplish a major goal of mine at close range on buff, but now trying to do it with Norma factory ammo and 570 grain bullets which I know the rifle didn't favor, (and I hadn't used factory ammo for anything in 25 years), because I tried them. Blah , Blah, got to camp and headed to the range. Gary asked what range the target needed to be set up, I said 50 yards. This was our first hunt together, so being the jokester that I am, couldn't resist setting the rifle on the sticks, but unknown to him did not chamber a round,....focused in, flipped the safety off and proceeded to do a camera worthy flinch to a click. The look on his face was priceless. After my wife and I I stopped laughing, I chambered a round and sent one downrange. 1 " high and 1" left. I felt like the luckiest guy in the world at that moment. Everything was going to work. Put the scope on, sent another round downrange, same point of impact. At this point all the trouble we went through to get there went away....and I accomplished my goal.
Moral of the story, if we can preach one thing, it would be to master your equipment. Know it in and out. Practice, practice, practice. There will always be something else that can go wrong, make sure you do as much as you can to eliminate the possibility of it being your rifle and your ability to operate it effectively. IMO, there are guys that can shoot 460's, 500's, 577's, 600's etc, but there are way more that can't. Figure out which guy you are and make your ego OK with it.....your EGO should be checked at the door as soon as you step foot in camp and prepare to go into battle with your PH. Much less about the caliber, more about the man behind the rifle and what he's done to "be ready".