Does he need a monitor? If not you could prolly do it as long as you didn't need to buy windows.
MSI RS480M-IL Socket 754 ATI Radeon XPRESS 200 Micro ATX AMD Motherboard - Retail
$85.00
pqi POWER Series 512MB (2 x 256MB) 184-Pin DDR SDRAM DDR 266 (PC 2100) Unbuffered Dual Channel Kit System Memory Model
$42.99
AMD Sempron 2600+ Palermo 800MHz FSB Socket 754 Processor Model SDA2600BABOX - Retail
$62.81
Western Digital Caviar SE WD800JB 80GB 7200 RPM IDE Ultra ATA100 Hard Drive -
$55.75
Subtotal: $246.55
Then you would just need a cheap case and power supply.
Ps. I chose this mobo because it had decent onboard video