lol hows it not possible? depdant on how he/she means they could have cut the wire on thier router power lead connected it to a car amp for house useage i done this once and it worked but not a bloody router adapter lol. But you cant se a router as an amp no ~_~
"lol hows it not possible?"
well the very first issue that arises is the fact a car amp runs on a DC (Direct Current) power supply. The power supply in the home is AC (Alternating Current) so unless the router power pack was powered by a magical plug which runs on batteries, then its instantly not possible. However, if this person has an AC->DC power converter as well, then thats the first problem solved.
The second problem arises with the cable, so unless as Zidio suggested, they have cut the cable to connect it to an amp, then it wouldnt fit, as amps use bare-wire connectors 99.9% of the time. So unless his girlfriend is going to cut/re-attach the connector at the end everytime they want to use it, it aint gonna happen.
Routers run off plugs that average up to 5amps or so, an Amplifier for a car runs off between 15-30 amps, meaning the fuse in the plug would be ****ed instantly.