Do you mean he's bought one of these? click here It's not necessary unless he wants to use it either as a print server or to stream music, the Mac laptop will just connect to his network via its internal wireless receiver.
To connect the Mac to the rest of the network, ie to share documents between machines, he just needs to go into the finder and click Network, which will then offer him the options. It's really easy, much less painful than connecting a Windows machine to a network.
I've got a network that consists of a Macbook and a Vista PC sharing files via an NAS server including the iTunes content and library with both able to stream via iTunes and my Airport Express to the speakers in my sitting room. The Airport Express also acts as a print server so that both machines print wirelessly via the network.
the only caveat is that you can't set up an Airport Express with Vista: Apple hasn't produced a Vista version of the Airport Express Assistant. You'll have to do that with the Mac. But all you do is run the assistant and tell the Airport Express to be a client on the main network rather than creating a network of its own.