Well, if she has that Facebook, that's evidence in favor of the claim that he did cheat on her. Especially if it was on the Wall open to the public.
But, if the girl that messaged her was the girl he cheated on her with?
I suppose I am too nice...
http://sify.com/finance/divorce-law...online-evidence-us-news-news-kg3hamhiehi.html
Seems you can no longer read the original AP study & corresponding article online, but I believe this website maintained most of the article.
edit: Link is to a report titled: Divorce lawyers: Facebook tops in online evidence, US
What people do on their off-time shouldn't matter.