Should I write on her facebook page in her conversations with him to bring him down a peg?
Or, should I just ignore it, like it doesn't bother me.
Neither. The first one is sophomoric game-playing, and the latter is disingenuous.
Contact is contact. Tell her you know she's contacting him, it needs to stop, and that YOU cannot live in a marriage where your wife continues to have contact with a person she had an affair with, and perhaps still is. Then if she does, you tell HER to leave.
