BT should provide a username and password for her ADSL connection, exactly as they did for you. Log into the router setup and find the bit containing your username and password, replace it with hers. Then look for a section called admin or similar and change the admin password to one of her choosing.
QUOTE: "To change the router settings, open your browser and type click here in the address bar. You will need to enter the configuration manager username and password. The configuration manager default username and password are both ‘admin’, although you can change this to your own choice. If you forget any changes you make to the configuration manager username or password, you will need to reset the router, returning it to its default settings.
Resetting the BT Voyager 220V (advanced users only). To return the router to its factory setting, you need to press and hold the reset button on the router’s back panel for ten seconds. The power light will show red to indicate that resetting is in progress. The reset process may take several minutes, after which the power light will turn green. The unit should now be reset and ready for normal use."