Agree on that first shot because it will be/should be a quality shot, especially on dangerous game. Plus the PH should be ready to starting pounding lead if anything goes wrong with that first shot. I think it's pretty much standard practice for the PH to start shooting if the animal doesn't go down, I know it was when I hunted there.
I also agree that a big cat that's wounded would be very scary, whether you were following up with a 10mm or a grenade! The lions I've been around are scary fast!

Dick