 The more we know, the more we can help
It helps a lot if we know what hardware is being discussed. Basically, brand and model of router. Is it Linksys, DLink, Netgear, Asus, Motorola?
You don't have to be part of his network. If you are getting signal from the router, you are able to connect to it. Probably. There is the possibility that he has it configured in a manner which does not permit setup changes to be made via wireless or remote internet.
Would it be a problem for his network if and when you make the change? Only for the time while rebooting the cable modem. UNLESS he has some very customized options for specific applications he uses. The standard normal settings that would commonly be used for his WAN are very unlikely to upset anything he does. Keep a record of the settings before you change them, in the event you might need to restore them.
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