In doing research for the .445 Supermag I'm come across information saying that .303 Brit can be used. But it does require some work.
Because the 303 brass body will be thicker he'll have to inside ream the case mouth once its been cut to length. The case rim is also thicker, so to avoid headspace issues, he'll likely have to thin it.
For these reason the 30-40 Krag may be a better option - But because the 303 can pretty much be found anywhere in the world, I figured I'd let you know of that option.
Oh and unless he gets / has a set of expander dies, he'll have to fire form them.