I've not hunted buffalo but it's next on my list. From an amateur point of view, I'll give my thoughts. Picking apart the story, there is a lot wrong and not much (if any) to your negative credit. Splitting up, first and foremost, was bad move 1. Splitting YOU up with the tracker, with no backup gun, was bad move 2 and could have been fatal. Advising you to shoot him on the shoulder with a 300WM is bad move 3 but I am not in a position to really say for sure. I would have been inclined to do what you did. Wait for the buff to look head on and try for a good shot. Bad move 4 would be cussing you out. He put you, the tracker, and himself in a really bad position. That's his fault, not yours. This isn't like you setout on the normal buff hunt with a .375+ rifle, PH with you having a big bore backup, tracker to assist. This just sounds like a cluster-you-know-what from start to finish.
There are stories in a lot of the safari classic books (Death in the Long Grass, etc.) that they talk about PH's allowing crack-shot clients to brain buffalo with 300WM, 7mm, etc. It doesn't get mentioned much. It likely would work. Is it smart? Probably not. I sure as hell wouldn't chance that 25 yds away. 100 yds away from a point of where you could take your time, go unnoticed, would be more appropriate.
In the end, it's good that it worked out. I'd be really thankful God was looking out for your interests on that whole debacle.