I do not believe the 458 Win Mag is barely adequate. Back in the late 1898 JR Rigby and Company introduced the 450 NE. It fired a 480 Grain bullet at supposedly 2150 FPS (although in reality probably in the range of 2050 to 2100 FPS. This set the standard for big game rifles. The are many other very similar NE rounds that fire a 480 or 500 grain bullets at the 2100-2150 FPS mark. Nobody doubts these rifles as "stoppers". The 458 Win Mag ballistics are very similar to these rifles. Most handloaders easily top the 2150 FPS with 500 Grain bullets. Hornady claims 2140 FPS from their factory loads.
I have chrono'd some very old Remington 510 grain factory 458 Win Mag and there ran about 1800 FPS. Definitely too slow for my taste, but these were very old. Maybe ammo from 40 years ago was inadequate.
Is a 500 Nitro, 505 Gibbs, etc better? Depends who you ask. They hit harder, have more recoil, and have more recovery time between shots.
I have shot eight buffalo. No shots (on my buffalo) have been fired by my PH. First two were one shot kills with a 416 Remington Magnum. (Both had insurance shots, but were not needed). The third took five shots in very close quarters with a 470 Nitro, I felt the first shot was very good, it just did not want to die. The Fourth received a marginal shot on the first shot (single lung facing me), second shot was too high, the third shot was solid, the fourth shot missed. This was all with my 470. We tracked it for quite a ways and found it layed up, we got reasonable close and I shot it 7 more times, it was still trying to get up until one of the last shots. The fifth one was assisting another hunter on a follow up on their third day of tracking. We bumped in to it accidently, joined their group. My PH and myself both shot it with 470 doubles at close range with seemingly little effect. Thankfully it did not charge. The sixth two a double lung shot from a 375, we trailed it for a while and I put another good shot in it with my 470. We found it dead on the edge of some 12 foot tall grass. The next one was with a 375 with a quick follow up second shot from the same rifle. It ran about 100 yards in the opposite direction we were, and died. The last one was shot with a 470 at about 25 feet. It was a high heart shot and never took a step. It is the only one I ever dropped in its tracks.
Once they are wounded all bets are off. I do not know if it is adrenaline or a will to live or revenge, but than can get tough. Shoot them properly with a 375 and it is all over after the first shot. Buffalo are tough and are not impressed by foot pounds of energy.
I think we owe it to ourselves, the PH, the trackers, and the game to be prepared as possible physically and in our shooting abilities. However not being involved in the incident I am unwilling to lay any of the blame on the client, the PH, the rifle, the tracker, the appy, or anything else. Once Mr Owain is laid to rest there will be plenty of internet discussion about what should have/could have/would have happened. Sometimes things just happen.
RIP Mr. Owain