If her's is regular domestic wifi and your are using cellphone type internet dongle I don't quite see how this would work.
If you have a phone or a laptop with built in wifi designed to connect to a domestic system such as hers, no problem. Start the computer's wireless networking setup, detect her router and apply the wireless security key. You then need for her to enable file sharing on appropriate folders, turn off her firewall and give you the computer name. You then need to disable your computer's firewall, go to My Network Places and Add A New Network Place -- follow the wizard.