the cheapest 8800GT is $111, and the cheapest 9800GTX+ is about $200.
so the 8800GT is slightly more.
I would still go for the 9800GTX, it looks nicer than the 8800GT, to me at least. I like them dual slots for some reason, and there isnt really any games out there that requires more than an 9800GTX at the moment, since i am able to run most games with an 8800GT.
But if he goes with the 9800GTX+, then later he can get another 9800GTX+ when it becomes cheaper and it will be faster than the 8800GT SLI.