For reference, to move my mouse in Windows from one side of the monitor to the other, it moves about 3" or less on a 1920x1200 monitor.
I have about the same sensitivity in games as I do for my desktop, so to turn doesn't require a huge amount of movement.
You really shouldn't be moving your mouse all that far to turn/etc in games.
Try upping your Windows sensitivity/movement speed if it's low, that way you will be able to get used to a higher sensitivity more quickly.
I think it depends on how far he wants to go with it and what type of games he is playing. I can't think of a single scenario in counter-strike where you have to turn your mouse more than 130* if your cordinated with your team. As such *most* players that play in leagues for a living use extremely low sensitivities. I don't want to say that you should drop your sensitivity as to where it is uncomfortable as ultimately you should'nt let other peoples settings affect whats good for you. There are plenty of amazing players classified under high sensitivity, and even some use mouse accel! Unfortunately it looks like Source might be dead competitively now with the new OJ box update.
I can't believe how terrible that engine is. It doesn't even use direct input, what a POS. I was about to say don't change your windows mouse speed because it will mess shit up, but any game that doesn't use direct input isn't worth playing.
Anyways, just use what is comfortable for you as thats what matters, don't worry about other peoples settings.