What's his budget? That really is the key factor here.
You'd probably see the most immediate impact from upgrading the GPU. The GTX 750 TI is the fastest card you can probably get without worrying about upgrading the PSU (Power Supply) or Cooling. It's around $150. It won't max out the settings of the most demanding games at 1080p, but it will get the job done with a few settings turned down. If you have a larger budget, the case should allow for a new PSU. The AMD R9 280X will probably keep you future proof for a while, along with a new PSU, like a 550W Antec, XFX, or Silverstone.
As stated, the CPU could be upgraded to to a 6-core FX6300 for $120, up to the 8-core 8350 for $180. More and more games are starting to take advantage of multiple cores. You'd still want to start with the GPU though, as that will give you the biggest impact in performance.
Lastly, you may want to consider a Solid State Drive, like the Samsung 840 Evo series. SSDs are a replacement for the typical Hard Drive (HDD) in PCs. They give you lightning fast Windows Loading and general usage, and games load incredibly fast. I'd make the upgrades above before touching the storage, and it will also take a little effort to either clone your existing drive, or to re-install Windows and everything from scratch onto the new SSD. Once you get an SSD, it's impossible to go back...