Almost always caused by one of two things....
1) using a bad password (
http://www.usewisdom....mputer/passwords.html )
2) the user going to some site somewhere that tells them it will let them know who has them blocked on MSN and all they have to do is enter their username and password into the site (giving the owner of the site full access to send spam from the account whose details where entered)
Get your neice to change the password on the account, use a decent strength password and don't type the details into any other site no matter what it promises to tell you.