We need much more information from you.
Does your server appear in in-game server list?
If it does, what exactly happens when your friend is trying to connect your server?
If your server isn't present in the list, ask your friend to connect your server manually, by opening a console by pressing Shift + ~ (aka Shift + `), and typing:
Code:
/connect 12.34.56.78:29070
Replace this dummy IP-address with your real, external IP!
Describe what exactly happens when your friend is trying to connect your server. Any error messages?
You must configure or disable your firewall, if any, in order to host a server.
You must configure port forwarding if you have a router, in order to host a server.
Here is some manual about port forwarding. While it's written for some specific routers, it can be easily used as a general port-forwarding guide:
Hosting Server: Windows
To find out how to set up port forwarding for your exact router, google "<router brand> <router model> port forwarding", example:
d-link dsl 504t port forwarding - Google Search