I have senco nail guns and some other brand brad nailer for finish work. All require an air compressor to make them work. My senco IV will hand 16 to 10 penny for rough construction on studs but it is heavy. The coil roofing nailer works great but only shoots big headed roofing nails which won't work for most other construction. I use the brad nailer for door casings and moldings. On some jobs it is more trouble to drag the air compressor around and hook it up than it is just to hand nail. Also the big guns are heavy and ackward to use and especially in a tight place trying to toe nail with one hand while holding the board with the other hand all the while hoping nothing slips and you get your hand or finger holding the board. When working overhead, the nail gun is still much easier than hammering though. The air hose adds lots of extra weight to it though. A not so big guy will struggle with a heavy gun when hanging and dangling overhead around rafters, etc.
It really depends upon what kind of carpentry work he does such as rough, finish, complete house building (where all sizes are needed), deck building, or roofing, etc. Post back with what he does. Also the bigger guns get quite expensive (few hundred bucks) plus that much more for an air compressor. I have a large compressor but still have to wait for it to recharge when shooting lots of nails with the big gun which gets aggravating. Post back with what he does, and with any major purchase like this that he would use every day, consult with him first and let him pick it out. I don't even let my wife pick out clothes for me much less a tool.
A cheaper and maybe better alternative would be an electric screw gun (a hundred bucks or so) or electric impact wrench. A screw gun works great on flooring, drywall, siding, fencing, and decks. One that holds strips of screws or coils (hopefully up to 3" to 3 1/2" galv for exterior decks and black for interior) will fill a lot of needs and he may find that he will screw more stuff cause it is easier and not use his hammer as much. He will get better holding power with screws. Plus all he needs is an extension cord. For long screws and for screwing metal roofing and siding, then the extra power of an electric impact may be warranted and he could change his tires with it also.
Good luck and merry christmas. You remind me of that commercial where wife brings home a new drill and some beer from the store. Hubby and other guy are on the roof and the other guy asks if she has a sister.