How do we know this guy isn't someone crazy who's trying to get back at her for rejecting him or something like that?
If she really did this crap and ruined his marriage then it would make sense that he would tell her husband right away and not wait for a pregnancy announcement.
Also couldn't she simply delete her pregnancy announcement before her husband sees this message? Wouldn't it make sense to message him directly with this information?
Just seems too easy to go on someone's FB and write some crap about how they've been trying to get you to have sex with them or something because for one reason or another you want to cause problems.
Although I do acknowledge that if her husband contacted him that he could theoretically produce messages she sent him to support his claims.