But a very good GPU a few years from now won't fit with the older components. He would likely need to replace the motherboard, RAM and PSU and possibly the CPU if the goes for an i3 like some have recommended. That kind of defeats the whole purpose imo.
A GTX 970 can go into a PCIE 2.0 slot with almost no performance loss. (PCIE 2.0 is 8 years old.) If an 8 year old slot isn't holding back a new GPU, there isn't much reason to expect 3.0 slot to hold back a GPU 3 years from now.
The suggestion is to skimp on the GPU, and go towards a better CPU. He could get an i5 for example. The motherboard, RAM would not be likely to need replacing. The only reason the CPU and PSU would go is if he wasn't planning on using them. He can get something like a 850W, and that should work great in even 3 years. The CPU like I said, he could get an i5 for example, which is a great CPU. If he's willing to extend his budget a little, he could even get an i7. Those CPUs will be good for a while. Most games are mostly held back by the GPU, with a few exceptions, and don't see much reason for that to change. Actually I'm expecting that to get worse. Especially as GPUs get better and more computing is done on there.
If he plans correctly, a good GPU will fit great with the older components. If he doesn't plan so well, then I will agree.