Every wedding I've ever been to has assigned seating. I've never been to one where it was a seat yourself affair. I'm sure weddings are evolving to a more casual environment and seating yourself may be in vogue now :)
Either way I would NOT comment on it at all. The person the comment was directed too has already expressed her wish for dirty laundry not to be aired on her wall.
I understand you are wanting to defend your friend but any comments you would make would only fuel the fire and provoke the nasty poster into "defending" herself against you on your friends wall.
Just let it go and the drama queen will move on once she sees she can not force your friend to engage.
Peace and Blessings,
T. B