Performance to dollar, the GTX 760 is probably the best value card for the money. Its slightly higher in price than AMD's comparable offerings, but nvidia tends to have better support and in my opinion better features like Shadowplay (Record your gaming!) and the GeForce Experience automatically tunes your games to optimal settings based on your system specs. Dual monitor setup is a breeze on nvidia cards. Note that with a dual monitor setup, he will actually only be gaming on one screen, it will not split screen a game between two monitors. If he was going to expand to a 3rd monitor and wanted to go with 3 monitor gaming, a 4gb model of the GTX760 would handle that quite good, playing at ultra settings on most current AAA games with a playable 35-60 fps. With a system budget of $1000 and needing to buy monitors and OS, $250 for the GTX760 is about the most you could reasonably spend without cheaping out on other important components (processor, motherboard, ram).
If you had to spend less than $250 for a graphics card, i would recommend the GTX 750 or the GTX 750ti. Both will play games in 1080p on high settings (not ultra) and a playable 35-60fps. Those start at about $150, but again, you're getting much more performance out of the 760s.
I hope this has at least been informative. Good luck with the build!