First of all, I think that you are totally in the right to not invite the girl who posted that message on fb. I feel terrible for your FI, because no matter how many times you tell him it was a joke and that girl isn't even your friend, it still hurts.
As for your "friend", I think you are right on. I wouldn't say anything at all, just sit back and watch. I would try to keep your distance from her for your own sanity, and I surely wouldn't share anymore details of your wedding with her unless you really wanted to. You shouldn't even have to be dealing with this crap!!