I get it. You and he likely have mutual friends on each others pages. Maybe even co workers who know about your initial relationship or marriage that won't react well to less conventional lifestyles. When she posts stuff with and about him - pics he is tagged to and comments on his page, those mutual friends and co workers tend to go hmmm WTF?
Then they contact you or him with questions you didn't intend to answer or expect to be bothered with when you're not the one dating her. Or worse just start avoiding you or treating you and he oddly. If you didn't expect it or want that kind of attention it can get sticky.
Make sure you and he are on the same tip regarding this. Adjust your privacy settings to hide it from certain people; you can make something invisible to individual people without making it so to all. Then have a sit down with her about it and see if it can be smoothed out.