Ah. well that was just the article that popped up on Yahoo. Still she had no right to impersonate him, or to change his password. I think by hacking they meant that she was in his account and changed stuff. If she hadw anted him off facebook she could have just banned the site from any of the home computers and set punishment if he got back on.
One issue is that apparently she is not the one who is his guardian. His grandmother has that designation. Since he's gone so far as to request a no-contact order over this, I'm assuming that she lives elsewhere and he must have been visiting when the incident occurred. Getting the grandmother in on it would be necessary to enforce such a ban (and, if she had wanted him off Facebook, that should have been done.)
My husband and I let our little one onto certain websites that have low-level socializing (she and some of her friends play fairies together). As she is a minor, we set up the account, know the password, and actually log in for her. If it ever occurs that there is a site that we aren't
going to let her on, then we share that information with each other, and keep an eye on the computer for that access. We would also let her Grandma and Nana know, so that she doesn't make use of their (albeit limited) computer access while she visits there.