RST calipers have a larger piston than 3i calipers. Otherwise they are the same I believe (anyone confirm?)

AFAIK RST hangers are the only real difference, as they need to accomodate the larger discs.

So he could use your calipers, he'd just need the 3i hangers.

Pretty sure on this info, but having never had everything side by side to accurately measure, dont take it as gospel