Different network. Have him paste this and tell you the result:
//echo -a $network $server
The first word would be the network name. If it's in the mIRC list of networks in your connect window, try connecting there. If it's not there, then you can try to join the specific server he's on, by using the 2nd_word in his answer in:
/server 2nd_word
If he's at the same network you're on, then the network is having a temporary split, and the servers will eventually re-join each other.
