Can't you simply tell the dial-up connection to "Never dial"? Then he can just manually connect with an icon when he wants to. Having windows set to automatically dial is so dangerous and irritating, I don't see why people like that feature at all.

Odds are, the thing that's trying to dial is a program in the startup group, the load= or run= lines in the win.ini, or a program in the run sections of the registry. Maybe it's even AVG or Ad-Aware trying to look for the latest updates. So your only defense would be to disable the automatic dialing or remove the offending programs.