How do you know this? Do you still follow him? Unfollow him and cut it off completely to allow yourself to heal. I’m not sure how long you haven’t been together, but it could quite possibly be a rebound relationship that he’s using to make you jealous and hurt you by posting her. It’s a common thing for ex’s to start posting on social media’s lots more and flex new partners to make the other jealous. It feels awful, I know exactly how you feel, but you need to prioritise yourself here and get this person erased from your social contacts.