At $500, it's doable. But it will probably take a little patience. As I said, out of the CPU/RAM/MB set, a Core i7 would eat about half the cost of the set. This is probably where he'll probably want to focus his search on Slickdeals. RAM will probably cost between $90-$110, depending on which kind of RAM he gets. For example:
G.SKILL 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Dual Channel Kit Desktop Memory Model F3-12800CL9D-4GBNQ - Retail
GIGABYTE GA-P55-UD3L LGA 1156 Intel P55 ATX Intel Motherboard - Retail
Intel Core i7-860 Lynnfield 2.8GHz LGA 1156 95W Quad-Core Processor Model BX80605I7860 - Retail
Total: $484.97
And the only thing on sale is the motherboard. But there are 2 socket sizes for the Core i7, LGA 1156 and LGA 1366. The 1366 is about $10 more, but it can only run up to DDR3-1066 RAM, which would drop the price a bit. It won't be high end but it you will get decent performance for mid-range parts.