Have him check the wireless rotuer settings. Your iBook probably runs in wireless “B” mode, which is an older, but still usable wireless speed mode. The router might be set to only accept connections from newer and faster computers – like the MacBook – which use “G” mode, or the newest, “N” mode.
He’ll have to look in his wireless router settings for an option to run in “G Only” or “Mixed B and G.” You want “B and G” because your iBook is probably “B,” and then he can still connect with his MacBook.
The other thing to check is the encryption. You didn’t say anything about it, but if the router is set up for WPA2, he’ll need to change it to WPA or WEP in order for you to connect, because your iBook can’t work with WPA2.