And that is why I said "decent" GPU and not "oversized" or "top-of-the-line".
Uh, you might want to throw a decent GPU in there instead of all those cores. His game probably won't push it very hard but it's important to have so you don't get creatively hindered by crappy hardware.
If you want to make a 3D game, just buy a PC with enough RAM (8GB+) and 4+ CPU cores. The rest doesn't really matter. Oh, and a good keyboard.
Feature-wise, the average GPU will be sufficient. When developing, one will also rather have an IDE that reacts and builds quickly than an oversized GPU. Only when using GPU-accelerated media creation software would a top-of-the-line GPU really matter (and even then, the CPU is probably more important).
Also, if you want your IDE to react and build quickly, you want to add an SSD before going overboard with 4+ cores. That'll make a much bigger difference.