I may have been just very lucky in the 3 barrel changes I have done on early Winchester lever guns ( and the one early Marlin for that mater also), all the replacement barrels ( someone else's factory built removals purchased at gunshows) screwed in and indexed and head-spaced perfectly. The boys back then must have had their jigs & measuring devises set for perfect batch manufacturing capabilities....and they paid attention using them!!
As already mentioned the mag model (button mag, 1/2 mag, rifle configuration, carbine configuration are all different dovetail spacing's) that your bud has might be a bug-a-boo to a plug-n-play changeover. If they don't match at the dovetail cut there is an easy fix without going thru the "cut a new dovetail thing". One of the guns I did a barrel change on had a buggered mag tube so I bought a new tube from Western Gun Parts and cut it to length to match the barrel and installed it in a few minutes work.