Are you guys both connected to the same router by LAN? I am definitely no authority on the subject, but whenever I visit my brother, this is how we connect to the internet, and we occasionally had problems like this when we tried to play online games together. We ended up having to use Hamachi and things like that when no solution was ever found. From what I remember from my GoogleResearch-Fu, the issue was said to be something about trying to have two different computers connect and transfer information from the same exact (insert jargon thingy here), and so nothing really was "connecting" because we were both using the same whatever-thing.