It is a manga graphic novel, right?
I have heard of that story and although it is WILDLY popular in Japan, those girls are much to young for that story. *sigh* I mean, that story is really heavy in terms of what is going on.
Do you know who bought it for her? If her mom approves of it, you can't stop her from having it but maybe tell them that you feel it is inappropriate in your home?

Sorry honey. If you have any questions about the actual book, feel free to pm me,