Windows 7 Network Map Showing A New Gateway?

I live in a student shared house which has a WiFi network set up. It has come to my attention that my network has recently gained a gateway between the router and the internet which is visible through the windows network map, myself and my housemates all connect to the internet through the router but it appears that this gateway is linked to one of my housemates, right-clicking the gateway brings a drop down menu with his computer name in the list.Since this gateway between the router and the internet connection has appeared there has been a noticeable performance loss to the network, being the only gamer in the house this has affected me the most in games having higher pings and dropouts all the time, although i understand that is also normal with using WiFi it has not been that much of a problem until now.I would like to ask how and why he is able to create gateway between the router and internet connection and how to stop it, i would like to also point out that the individual love his torrents and has possibly found a way around the bandwidth limiter in the router, he has currently denied any foul play but has tried other things in the past.