Well, if he is going for 4K gaming, depends what type of games he wants to play. The 4K gaming card is $600 by itself, just something to think about. You can also get a smaller card, and drop the resolution of the game. It won't be maximum for the screen, but you can always upgrade early.
The system itself, will probably be $600-700 itself, and then the GPU will be $200-450 depending on how much you are willing to spend. Clearly if you want true 4K gaming, you are looking at around $1,300. It's up to you.