Mark should not participate in the trashing of Christa. What the boys are doing on the private Facebook page is very wrong despite the fact that it cannot be seen by her.
There are multiple reasons people post bad things about people on social networking sites. One of them could be because they find it humorous to make fun of people, when really, it isn't.
The right thing for Mark to do is to tell someone. He should tell regardless of how the boys react to it.
