Can facebook be used to prove cheating?

[Jo]

Definitely something fishy going on here, she would have to be a complete loon to do this if she what she is saying is a lie. Maybe it is true or maybe something happened once and now she wants to punish him. I would not just accept what he says, ask him for his facebook password, if he is innocent he won't mind giving it under these unusual circumstances, don't let him turn this back onto you by saying that it's you that does not trust him, you have a valid reason to want more information from him.
If he is innocent then he should be avoiding this woman at all costs, no matter how involved she is with his family, why would he want to be friends with a woman trying to wreck his marriage?
You must be going crazy. If you can't find a better solution then I suggest installing a keylogger on his pc/laptop to see if they have any communication after he arrives home. Good luck.