Dell dimension 2400 series, with the cheapest possible options, runs just a bit under $500. Then get a PCI FX5200 card for $50or less and you're in business. They throw in a 17" LCD monitor for free which could even be sold for enough to buy a video card since he already has a monitor.
My point wasn't to suggest this was the best course of action, just that it was possible and an alternative to spending hundreds of dollars upgrading a legacy system and that upgrading to be able to run AH2 isn't impossible.
If it was me, I'd go with something like this:
ABIT NF7-S motherboard - $130
AMD Athlon 2500+ (Barton) - $80
256 meg crucial DDR PC2700 SDRAM - $57
GeForce FX5700 - $98
Reuse all the other parts. The motherboard has decent onboard sound and LAN.
Might need a new power supply and case, but those can be had for well under $80, $100-120 for high quality parts.
For the absolute cheapest, he could get either a PCI or AGP FX5200 card (under $75) or GeForce2, or GeForce 4MX, and one more stick of 128 meg memory for under $50. The kids in my old neighborhood were charging $15-$30 for mowing lawns so it's not impossible.