With the 9600xt's at $150 now, they're not bad for the price, especially if you get an hl2 coupon. If you bump it up to $200 though, you can get a 9700 which would be nice.
As far as whether the card will be compatible with the mobo, which card is in the computer now? That might give you some indication. He could also download aida32. It should give him the model of his mobo and the agp version/supported speeds.