If she said something about somebody else in a public way (yes, your "friends" probably count as public), the person being spoken about has multiple options:
1. Suck it up. People say mean things; get over it. (Side note: my guess is this whole case boils down to "contempt of cop", that horrible crime of not showing a person who has authority over you enough respect). Students don't check free speech at the gates and especially don't at home.
2. Contact FaceBook about a violation of its terms. FB could delete said post. I think FB's 13 year old policy is stupid (yes, the underlying law is also stupid, but FB takes the stupidity to new heights), but it is their policy. The principal could have just gotten the account nuked.
3. For a public posting, involve the parents. It is, you know, their job to raise their kid. If a parent decides they value freedom of speech over respect of authority, well, see #1 above or #4 below.
4. If the comment is truly libelous (and, remember, to prove libel you have to show harm was done[1]), you always have redress in the courts. But, again, the inequality in the relationship means the principal doesn't have to bother with that.
We want adults who question their leaders, yet our schools (including, sadly, many colleges. Just look at what happened at Berkeley) do everything they can to crush any leadership questioning. Is it any surprise most adults don't care about what their leaders do? They have been trained from a very early age that they have no control over "the man". Why would that change after college?
Now, I'm not saying what is right or wrong here. I'm simply asking what people think should be done. How do people want it to be handled.
Also, shouldn't Facebook be deleting her account? She's only 12, and you need to legally be 13.
http://www.facebook.com/help/parents