If you don't already have the HD 7850, I suggest you grab the HD 7870 instead. It will give you a significant increase in performance. You should be able to play most games on ultra settings with this build.
That's not going to help in games. Games are typically bound by not having enough GPU horsepower. Getting an i5 isn't going to help at his budget. Rather, he should go with an i3 to save cash. Then use that saved cash to sink in a high end GPU.
The i3's are plenty fast. Just get a Z75 or Z77 based board to go along with it and you can upgrade to an i5 3570k once you've acquired the necessary funds.