I wouldn't think it'd be much more difficult. You'd have to be careful about more area of stitching, to be sure it isn't too tight anywhere, but, really, as long as it's tight ENOUGH in just two spots, there shouldn't be a problem.
One thing I might consider with a barrel that long is speed of presentation, if he's using it for competition or even hunting. (Pretty sure combat use is out of the equation with a barrel that long - also, is that a .41 Magnum revolver, or a S&W MODEL 41, which IIRC is a .22?)
With long barrels, you might consider some sort of cutout or split to reduce leather-clearing time. Along the front edge of the holster if it's a strongside, along the rear edge if it's a shoulder holster or vertical crossdraw. Front crossdraw should be fine as it is.
Also, if it's the big revolver, it's gonna be HEAVY, so I'd make a large, sturdy belt attachment.
Might even consider making a reinforced belt, or suggesting that he use one, anyway.