Hoppdoc, if I were in the skin of your friend, I would not hesitate to use a QR scoped 8x57/12-12 drilling on a leopard.
It can be as accurate as any bolt or single shot rifle for doing a perfect one shot kill, the caliber is just about ideal and in case a follow up is needed and your friend still wants to go, he can have up to 3 shots without reloading, wich is undoubtibly better than only 1 of a bolt rifle, plus the one or ones of his guide.
I have seen top qualified ph´s missing decisive shots, and this can happen to any one at any time, and a front running leopard is the perfect candidate to be missed.
A bath of 000 pellets thrown at the right distance may not penetrate the skull, but it would be like be stopped by an invisible solid glass.
And one more advice, it is better the keep one´s skin than to loose the leopard´s. So tracking a wounded one in the night is a rashness job for lunatics only and it is not under the rules of true professionals, whose slogan says: NEC TIMOR, NEC TEMERITAS (translated from latin is: No Fear, No Rashness).

Regards and end of transmission.
The Preacher