the toggle link is stronger than the old myths and urban legends of he past would have you believe, but i would stick with 240 and 250 gr loads, loaded to factory specs
maybe an occasional [ for hunting ] 265 gr load

He should get inside an 1873 and take a look at how it locks up. It is NOT an 1892 by any stretch of the imagination. With modern steel and such it's likely stronger than the originals, but it's still a toggle link. If he wants to hotrod it he'd be better off picking up a '92 instead. They are a fascinating reproduction, but they are still the old toggle link at heart - not designed for high pressure modern loads.

YOUR brother is looking at this wrong, Bill. That rifle will take standard factory 44MAG loads from 1300-1400 and goose 'em up to 1800 strictly from the longer BBl on the gun. This alone produces about 70% more muzzle energy. There is no substitue for velocity not for barrel length to get it. Also , when you noted the word 'heavies' do not know if you can get 'em in the gun as the '73 takes a cartridge LOa of 1.590" MAX on everything else and I suspect that includes the 44MAG cartridge.
That rifle was proofed with to CIP specs and they tend to be a bit higher than our SAAMI specifications, but for dangsure I would NEVER push it past that.

If I had one I would be glad just to load a 200 grain RNFP like I do in my 66 only instead of 1200 with the 66. A steel 73 chambered in 44 mag I would have my load doing about 1500 or so and think of it as a souped up 44-40 and be glad.

Bob