It really depends if he has previous motorcycle experience or not. It also depends what he plans to do with the bike:
If he just wants to vroom vroom, show off, and as Clarkson on Top Gear says "Feel a little naughty", then get a 600. Honestly, he'll probably be able to ride it in a week or two after testing it around.
Now, if you plan to actually do something w/ the bike legally like I am, I'd suggest you start smaller. The reason is simply because "can you ride the bike to it's limit?". Seeing as how riding is a sport, with any sport you need to start smaller. I didn't start benching 2 plates per side on day 1. My runner friends didn't start doing marathons on day 1. My track friends (myself included) didn't start on supercars or 300+ HP cars for their first cars. Why? Well, for the physical examples, you just can't. You won't be able to reach that limit...period. For the car example, you, once again, won't be able to reach that limit.
Now, if you're an adrenaline junky, feel free to do it. I would if I had the balls but I don't. Going in a straight line is easy, but how fast, how much lean, how much throttle you can open up when exiting a turn, and many other things have much lower limits on smaller CC bikes. For example, the other day I tried to make a right handed turn at 60 km/h. I opened the throttle WAY too much and early when exiting and the rear wheel shook around; needless to say I got a little scared. On a smaller bike, that probably wouldn't have happened because of the throttle limitations. Smaller bikes also generally weigh less. And mind you, I ride a GS500F that everyone here laughs at.
But above it, it hurts a lot less if you drop a smaller, less expensive bike, than bigger, more expensive bike.