First get some kind of external drive large enough to "hold all her stuff". It could be a hard drive or a USB flash drive.
Then, make copies of all "her stuff" onto that drive.
Then create a NEW account with the username and password she wants.
Now, LOG INTO her new account.
Next, connect the backup drive (see special note below), and copy things from it to where she wants them in her new account.
NOTE (and this is important):
Before she copies anything, you must first take action to prevent "permissions problems".
Do this:
a. click ONE TIME on the external drive icon to select it
b. type "command-i" (eye) to bring up the "get info" box
c. at the bottom of the get info box, she should click the lock and enter her new password
d. next, put a checkmark into "ignore ownership on this volume"
e. finally, close the get info box.
She should now be able to copy the files from the "old account" into her new one, and all the files will "come under the ownership" of her NEW account.