His machine should have a sticker on it with a valid Microsoft code, and that can be entered in the home page of system (right click on Computer icon on desktop and choose properties).
If there is no sticker, a call to Microsoft is probably the first port of call. It's a freephone number.
This crops up quite often. Manufacturers install with a single code, leaving the sticker on the case for validation.
If there is no sticker, a call to Dell should resolve the problem.