I think you're going to be okay with that card as long as he only plays SC2. It may not be great on Ultra, but I'm betting it will still get ~55FPS on Ultra (no AA) on a typical 1v1 game online. Very playable.
I think SC2 is more demanding of your processor than the video card, not saying you don't need a decent GPU, but it's not as demanding as say BF3 or Metro. I don't know a lot about the 260, but I think you'll still be ok.
If you are going to get a new card, and only want to play SC2, and only using one monitor, I wouldn't recommend spending over $100 on a new card. You simply won't need much for that.
My personal comparison comes from SC2 play on ultra at no less than 60FPS with SC2 on one monitor and two other monitors playing video and browser simultaneously, done with a sub $100 ATI 5770. If I have the right equivalent in Nvidia in my head, your 260 is a little slower, but comparable to that 5770 if you remove the other two monitors.