Most games that will come out will utilize quad cores. the i3 is a great processor, don't get me wrong, but I don't think 2 cores will cut it. This is planning to last him around 4-5 years, so I wanted to get a good processor that will last him, and he can upgrade the GPU more easily than the CPU.
It is just as easy to upgrade the CPU. the i5 and i7 share the same socket at the i3. If he goes with an i3 and a better GPU he will be able to play games on high/ ultra now. If he goes with the i5 and 7770 he will only be able to game on mostly medium and some high settings for about a year before he would need an upgrade.
To me it makes more sense to build the best possible gaming system for now and then upgrade the CPU later when the game technology needs it and not have to worry about the GPU.
Its basically just between playing on medium/ high now or playing on ultra now.