Ex's Facebook Message Confuses Son

My son saw a message on his grandparents' facebook page last night that confused him.  There was a message from the ex-girlfriend on their page wishing them "Happy Anniversary Mom and Dad."  That confused my son.  He's like, "What is this? I don't understand.  They were never married."

My husband told him that it wasn't important enough to get upset over.  We didn't tell him it's the ex's way of getting a dig in, that she's interfered even in my stepkids spending time with their grandparents this past year and hasn't sent them pictures either.  Her intent was to jab at us in front of all the family and friends online.  My son got the jab.

I would think leaving a "Happy Anniversary, Grandma and Grandpa" on behalf of my stepkids would be more appropriate since my in-laws are only related to her as the grandparents of her children (not by marriage or even a great, loving relationship) and she's done a botched job of even that this past year or so.  She was a pick-up in a bar, not a wife or loving girlfriend.