That’s extremely immature and pathetic that he did that to you. I’m sorry you’re going through this. The best thing you can do is act unaffected by it. Clearly they’re both trying to get under your skin. Very, very immature. I know my ex does subtle things to get my attention on social media, but this is really bad. You’re being the bigger person by not retaliating.
She’s going to realize that this guy can’t give her what you can. You have 6 years of history. I would do NC, it’ll probably drive her crazy since it looks like she’s looking for attention. He sounds like a rebound, because why else would she be rubbing him in your face? If it’s truly love, no one else’s opinion should matter.