Shouldnt matter. The Router in question should be backwards compatible and not only give out a G classed signal.
Example: I've got two Linksys wi-fi cards for my laptop. One is matched (exactly) for my WRT54GS router (the card is 802.11G and supports Speedbooster). The other wi-fi card is 802.11B, plain 802.11B. No matter what I do with it (updated its drivers, double/triple check I'm inputting the password correctly), the 802.11B card cannot and is not able to connect to my WPA/WPA2 encrypted network.
My point is, when all options are said and tried and done, if you can't get your friend's laptop on, it's not going to get on.
However, if you can provide us all with the specific name/model of your friend's wi-fi card, we can troubleshoot it a bit better and try and figure out for sure if he's SOL with your network.