Good point. It all depends on DNS and other settings. This is why I have asked for ipconfig /all on every machine. This would help know about that. He seems rather savvy. He has also stated he can browse the server, etc.
Again, go backwards. Look at the network settings on the server, then make one client the same in static or reservation and go with that.
Personally, I think you have to stop sharing the router. The ISP can't be that expensive. If he can afford it, just tell the other guy he has to get his own.
