When you send someone a file, you actually turn your computer into a server, tell him what your IP is and ask him to connect and get the file. Now many things can prevent him from connecting. Perhaps you gave him a wrong IP address (which is what "/localinfo -u" attempts to fix); or there is a firewall blocking incoming connections on the port you're listening to; or it could be a router not forwarding his connection request to the correct computer.

You can try to trace the problem and solve it, but since it's not your personal computer why not just go the way around: ask your friend to turn his computer into a server and connect to him on any port you wish (e.g. 80, that's likely not to be blocked where you are). To accomplish that, ask your friend to type:
  • /dccserver +s on 80
Now, ask him for his IP address, and to start sending a file merely type:
  • /dcc send IP:80
Yep, that's it. Now your friend can disable the server manually by typing
  • /dccserver off
Good luck...