@JonHig,
Because her ex is probably posting more, and she is liking similar posts.
The algorithm for deciding who are the people in the nine photo grid - that algorithm is proprietary, but the main things that FB does is, it serves you more content that's similar to what you have provided a social signal about in the past.
Case in point. I like Boston Terriers. I think they're funny. I am delivered with a ton of BT posts, ads, groups to join, friends to make, rescue groups to follow, etc. If I got tired of them and moved onto Labrador Retrievers, eventually FB would catch up and start to deliver me a ton of Lab posts. Further, if I got tired of BTs but didn't fill in anything else to replace them, FB would still serve me BT posts because Facebook would not have yet received any contradictory social signals.
Want to change who appears in the nine-person grid? It'll take a while but -
- Pick someone to replace them. Choose a person who is active on Facebook, as inactive people won't hit the algorithm quite so hard. Let's call them your new favorite.
- Add that person to your 'close friends' group.
- Go to their wall and post on it a few times every week.
- Like everything they've posted for the last 2 weeks or so.
- Start liking everything they post, and comment on and share that stuff, too.
Facebook will catch on after a while and will flip that person into nine-grid status. I can't tell you how long this will take (or who in the nine-grid they'll bump out), as a lot of factors come into play, including how many other friends you have and how much you interact with them, and what this new favorite shares that is possibly in common with your other friends. The strongest social signals will come from liking and sharing unique content that your new favorite is releasing.
Anyway, eventually, Facebook will figure out that you've got a new favorite but, like I said, it takes a while and the whole thing is complicated by Facebook's enormous size and the tsunami of data that we are all hit with on there, every single day.