I would recommend Woodleigh 410gr softs for buff. I've shot a couple or three over the years and I'm yet to fire a solid at a buff. A Woodleigh bonded-core soft will do everything a lesser-brand solid will do IMHO.

Like Mark I would also suggest your brother use the .577 with you as back-up with the Rigby if the quarry needs subsequent shots and he isn't quick enough. The relatively new Woodleigh 650gr .577s designed for the .577-90-650 nitro cartridge would be an absolute gem, in either steel-jacketed solid or soft-nose. As you probably gather, I would strongly recommend the latter!

With either of these cartridges and Woodleigh softs, just aim for the point of the shoulder. Game over!