I will agree with Cred, I would be more afraid of a 12 year old that did not have a interest in it. And would not go to hide it.
I guess my question would be as a parent why they could look at it for 3 months without you knowing,
Computers need to be in a public place in the home, so that when they are on it, you can see it.
Also you as a parent should have been checking on a regular basis the history of the computer.
After that, also there should be content blockers on all teens computers.
So block this from her computer and that is a done issue.