The GF6600GT is not hugely faster, but it's a solid 10-25% faster in most low res situations where they would both be forced now.
The other nice advantage of the GF6600 series is that he will now have access to the greater number of games that are starting to use SM3.0 as the minimum requirement for their games (like the new Splinter Cell, RainbowSix Vegas).
The R9800Pro was a beast and a good card with a long life, but at this point the GF6600GT is a solid performer and a better card with no drawbacks in comparison.
If he has the cash though I'd suggest a cheap GF7600GS or X1600-X1650Pro as a better upgrade for longer term enjoyment since they are more efficient at new games (especially using things like HDR), and usually about the same price or within $5-10 of the the GF6600GT.