Not sure what this has to do with Slackware, but whatever.
You need to tell your friend to forward the required port(s) from their WAN connection to the appropriate IP on the internal network.
The setup for this is different with each router model and brand, so you will want to go to a site like
PortForward.com to get information on his particular device. Of course, the manual for the router would work just as well.
