The easiest way to see if you'd get a boost is if you notice any slow downs in games. If you have low FPS in games you play, then the extra GTX260 will help. If however everything feels smooth, there is little point in adding it in. It will only increase heat, power draw, etc.
Perhaps psycho, but will he see it? At 1680x1050 his one card might be getting around 90FPS. (split the numbers listed.) Thats still faster then his 60hz LCD can show. How much more speed does one need? As mentioned there are some games that will need the extra help, but as I mentioned unless he's feeling the need for more speed he doesn't really need the second card.