Yeah, that would be a significant upgrade. It's hard to say how often he's CPU bound, probably still most of the time he'll be GPU limited, but BF4 MP, for example, is notoriously hard on CPUs and that upgrade would likely make a noticeable difference.
Just make sure he updates to the latest BIOS before swapping CPUs, motherboards that launced with Sandy Bridge (2xxx CPUs) weren't compatible with Ivy Bridge (3xxx CPUs) without a BIOS update.
Also, if he has a Z68 or Z77 Mobo, he'll be able to overclock if he gets a "K" processor and an aftermarket heatsink. It'd be worth getting the 3570K even if he doesn't want to get the cooler and OC right away, at least he'll have the option down the track. If it's a non-Z Mobo, just save your money and get the non-K processor.