I’m 6’4″ myself and my first bike was an ’08 Ninja 250R. It’s a GREAT bike, and it can be a lot of fun to ride, but the ergonomics simply aren’t ideal. If I’d known more about riding and the ergonomic issues that would come up, I’d probably have gone for an SV650 or something. I’ll sum up a couple issues I’ve had:
1) Your knees will be too high up on stock footpegs (I got lowered aftermarket pegs) to fit in the knee grooves in the tank.
2) You will be really tall relative to the bike. The Ninja 250R is SUPPOSED to be relatively upright, but because your torso will be longer than expected, you’ll end up in more of a sporty riding position. This means less wind protection and more work for your back and leg muscles.
3) Even with the rear brake and shift levers dropped all the way to their lowest positions, they’re too high. This is especially bad for downshifts because you can’t keep your foot on the peg.
Now about your friend’s offer, Kawi doesn’t make a Ninja 600 these days, so if it’s a newer bike, it’ll be one of two: the Ninja 650R or the Ninja ZX-6R. If it’s the Ninja 650R (649cc) and your friend is a responsible, cautious, and safe individual, go for it. If it’s the ZX-6R (599cc), don’t do it. The ZX-line bikes are all race bikes, and while it has 50cc less in engine displacement, it makes almost double the horsepower of the 650R, has a much more aggressive setup, lethally powerful brakes, blah blah blah.