How to deal with someone who constantly targets, defames and insults another individual on FaceBook?

[szhpaki]

The target of these insults is not even on FaceBook but is extremely well known among the friends of FaceBook abuser. This is creating bad-blood in real-life community with potential for serious social consequences.

I am not a FaceBook user but I have seen many seriously disparaging posts and comments which could cause a very serious situation if said in person.

Are there any rules of FaceBook or international cyber-laws that could be used to stop this on-line bullying?
I feel I need to add some clarifications.

I know about the posts because they are spreading like jungle-fire among the people who I, victim and abuser, all three of us know and have social consequences namely defamation. All of us are adults and cannot go to any school administration since we are all adults, not in school and illiterate of most of internet.

Abuser apparently knows nothing about internet but how to use FaceBook. Yes, he is jobless or semi-retired. I have been told he seems to be updating 24/7.