This is the main question. Is he using the "AOL for Broadband" service, which is just an internet service that runs ON TOP OF your broadband connection? OR is he running AOL Broadband, which means the connection is supplied by AOL?
If it is the former then you want to set up the base station to work with the cable connection, and ignore AOL when you set it up. Then you set up the PC as you have it now and it should just work normally. The computers over airport won't be able to use AOL unless they have the AOL software installed.
If you have the latter then you're going to want to call AOL tech support (when prompted press the button for Mac support) and talk to them about it. My cousin who has AOL dialup had to call AOL for them to explain how to set up his base station to work with AOL.