Is it a Motorola modem?
If so, you can access the diagnostics of the modem from any PC that is connected, even through a router, wired or wireless.
Click this link: http://192.168.100.1/config.htm
If it is a newer Motorola, then you should see the reset button on that page.
If not try typing 192.168.100.1 into your browser.
If you can reset it this way, then he'll never know.............it will just look like the modem reset for some reason.
BTW; just hit "Restart Cable Modem" , don't reset the defaults.
Although that won't hurt anything, it will just take longer to synch.