Disconnected Router
 

My friend's router's wired network goes offline after he plugs in this specific computer. It runs fine with the other two computer but once plugged, it simply disconnects all the others. This is probably not enough info but what are the probable situations that could cause this?

Any help would be appreciated.

Has he tried using a different NIC?

Did he try using all the ports on the router? Did he try another cable for the problematic computer? He could use 1 of the cables used for the other computers.