Some ISPs have a backwater way of "black hole routing" people to prevent excessive broadcast traffic. My provider does this. I have directional wireless Internet service. I cannot directly communicate with my neighbors on the same service, because the router at the tower blocks the traffic.
I personally think it's a terrible way to handle things, and it breaks the Internet in a very bad way...
If something like this is happening, your friend may simply be out of luck. If your provider is blocking the traffic, there may not be anything you can do.
Only exception - and a nice way around my ISP's blocks, is if you're on a small ISP that doesn't secure their shit properly, you can setup a private subnet like 192.168.100.* on your router's external interface, don't run dhcp on it, and just assign the same range of IPs at your friend's house.

That actually would work, with my ISP, because it would never hit their router. Just throwing that out there, not that you'd actually want to do that.
Cheers,
Kirk