It depends on the resolution/games you play but in some games you'll notice bottlenecks.Since Q6600 is a good overclocker(especially if you have the G0 version) you can easily get 3.0GHz(minimum) out of it
I would whole heartedly recommend you to get the ti. It will bottleneck, but it will be very very small. You should be easily able to squeeze out more 95% of performance than you would with lets say a i5 2500k. Although i must add, lower resolutions and cpu intensive games can give you a little less performance. Still, q6600 is still a great chip. Even without OC,your friend should do fine.
I see, well its not OCd and the guy who has it wont anyway, so would you advise him not to upgrade to a 560 Ti?
That is quite unfortunate as his CPU has much more potential just waiting to be released.
While even an earlier B3 chip can usually hit 3GHz without to much of a fuss, a newer G0 should be able to hit 3.2-3.4GHz easily and 3.6GHz+ with a little work.
All in all, even with any bottlenecking that may occur, a GTX 560Ti is much more powerful than his current 8800GTS.
If his system is lagging with newer games upgrading the GPU will give him a much smother experience.
This will hold especially true if he is playing at higher resolutions and settings where a CPU bottleneck would be minimized.