Where's the typical ARFCOMMER reply ".223 will do it, you just have to place it correctly!" ?
I've never hunted Cape Buffs, or been to Africa, but I'm well read and an experienced hunter so YMMV.
I say go as big as you feel comfortable going. .375 H&H is minimum in my book for Cape Buff. It'll certainly work though.
Is he taking a double rifle or a bolt gun? That would affect my choice. I'd be more comfortable with the .375 H&H in a double than I would in a bolt gun.
I'm going to SA in August but will be doing plains game only this time around.
Tell him to have fun, and if he can, get ahold of "Boddington on Buffalo" on DVD. (I think thats the correct title.) It's a great DVD by one of this generation's hunting greats.
Is this his first trip to the dark continent?
ETA: .45-70? You've got to be kidding me! I love the round but damn....