>>>Can I ask what a dongle does don't think he has that
Unlike modern laptops, desktops don't normally have the 'radio transmitter/receiver' built into them which is required for communicating wirelessly with a router. A dongle provides that additional component. (Actually, I should really have referred to it as a 'WiFi adapter', as the term 'dongle' is also used for a device which connects a computer to the internet using mobile phone signals).
Once he's got a WiFi adapter he'll need to know the network key (= password) used by his router, if one has been set. If he doesn't know it, or know how to set a network key, his ISP should be able to advise him or (if you tell us the make and model of the router) we should be able to help.
Connecting the computer is then simple. Here are the instructions for XP:
http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/wlan_client_connect.mspx?mfr=true
but it's basically the same for all versions of Windows.