Is he using the original 802.11b Airport card or the newer Airport Extreme 802.11g card? If he is using the original Airport card, it can only do 802.11b and if the WRT54G router is set to only allow 802.11g (54Mbps) wireless connections, then his PowerMac won't be allowed on the network. Go into the router setup and make sure it allows 802.11b/g connections (both).
I'm assuming he has already entered in the WPA passphrase. This will be necessary for him to join the network. If you are doing Mac Address filtering, make sure to enter his Airport ID into the list. If you are not doing this, then don't do it. Mac Address filtering doesn't afford any protection against crackers trying to break in anyway.