It depends on your router. With mine, I go into the router config. Open a browser and type "192.168.1.1" and login. Usually the default is admin admin. Then from there, you need to find the setting that forwards all traffic on a certain port to her computer. Usually the setting is under application forwarding or somewhat. Whichever port the vnc server uses, set that port to forward to whatever her local IP address is (find this by typing "ipconfig" into a command prompt). Other than that, you could probably easily Google a tutorial on how to set up VNC servers and clients.
For those about to hack, I salute you.