I flogged a YZF600 for a couple of years before getting the FJR.
Wonderful bike. Perhaps the most underrated sport bike (also small sport touring, which is what I used it for). The Katana is slow and sloppy (and brakes suck!) in comparison. Huge difference. Steer away from the Kat if the YZf is considered.
For his size, it will fit him PERFECTLY. I'm 6'2"ish and it was a "little" small, but not bad at all.
Mods for sport touring...soft bags, tank bag, Heli Risers, National Tall Shield, heated grip inserts, gel grips. I put a Yosh pipe and a jet kit on it as well, but both are not really needed. The brakes are superb on the YZF. Handling was really, really flickable yet stable and forgiving. Power was good--a little slow on the lower end, but had good midrange and high end pull. 130mph was all I had it up to, and it felt great at high speeds.
The YZF suspension is frankly A LOT better than the FJR. Very adjustable and never felt the need to modify springs or valving. Just clickers and preload is all that is needed.
Truth be told, if I could have afforded it, I would have kept it for short, spirited trips.
Never rode an FZ6. Seems very tall and "in the wind" to me. Plus the suspension and brakes are only so-so, or so I've read.
However, me thinks the YZF is perhaps a good second bike for a newbie. They are deceptively quick and nimble--easy to get in trouble with. If he's mature and has a good head about himself, then I say go for it.
My wife is about the same height as your cuz and has a Honda Hawk GT (650, v-twin). AWESOME first bike. Less than 400lbs, not too powerful, good brakes, very nice looking, and the following is HUGE and support is as easy as asking the Hawk mailing list.