Alright, so here's what you want to do. Show him this post:
1) What sort of use will this computer be put to? Light gaming like world of warcraft and call of duty, heavy gaming like battlefield 4 or starcraft, streaming / editing videos, ect?
2) Go to youtube and watch a video on how to put a computer together. I like the ones by newegg and NCIX the best.
The thing is that's an awkward budget for what you're looking for. If you didn't need a monitor and everything else, you'd have enough for an incredible gaming computer that would trash everything out there. Throwing in the monitor and such, you're still going to have a good PC, but it's going to have parts in it that aren't designed for upgrading - there's no point in buying a $250 graphics card if you're going to replace it when you can.
I would consider buying a REALLY nice monitor / keyboard / mouse setup. I mean a mouse that will last forever, a mechanical keyboard (NOT a plastic piece of... stuff... labeled as a 'gaming' keyboard), and a 120Hz monitor - BenQ's by far the best out there. (Asus has 144Hz monitors, but that to me is a gimmick, as you can't hardly tell the difference and the color of BenQ's monitors is far better.) That'll cost you about $400 for the monitor, $120 for the keyboard, and $80 for the mouse.
Then you can get a computer that's designed for upgrading - an overclockable processor and motherboard, a decent power supply, and a graphics card that will play games decently but isn't a powerhouse. That way, further down the line, you can buy an aftermarket cooler and overclock the CPU, and buy a better graphics card to max out details.