I too generally prefer nVidia cards, although in part because I believe more non-game applications will be written to take advantage of CUDA. Nevertheless, there are price points where AMD/ATI has the advantage for gaming. You might look at, and show your friend, the THG monthly article recommending the best video card at different price points:
Not surprisingly, in the cases listed by boris, the performance increases as the cost increases. However, note that the increased performance relative to "fps" may not even be noticeable, but might be beneficial at a higher resolution or when games demand more power. It is a personal decision if each step increase in performance is worth the added cost.