Which version of Netscape? On which OS - Windows?

It probably lives in a file in the user profile directory. In the address book window, under the tools menu there may be an "export" option. It will export in several formats - you probably want CSV or text, though it will depend on why you want to back it up (transfer or later recovery).

If she's thinking of upgrading to Mozilla/Thunderbird (yay!) then that will automatically import a Netscape address book, so no need to do anything too fancy.

Hope this helps.
Chris