She may have been hacked via a website..
Some people sign up to websites giving there login information, then the "company" will go on for less than 1 minute to post advertisments..
or
She signed up to a website without thinking giving her login details which leads to being hacked.
But like Daffy654 said; if she can get in, change her password.. I would also suggest, setting up an alternate e-mail address - so if she ever forgets her password, or gets hacked again and them changing the password.. All she has to do is go to "forget password" then follow the instructions.
How to setup an alternate e-mail address? Go to https://account.live.com, login, then go to the alternate email address, enter perhaps the parents email address.
Sorry if this is superly confusing ;-) if you want a bit more "simple" just leave a message