It sounds to me like a rebound too…
Im dealing with something a little similar although i know more info about my exes new relationship. I dont think you can really be sure exactly what it is with the little information you have. It does sound though that she may be enjoying the attention from him, RIGHT NOW.
Someone once told me “A new toy is more fun than an old toy” but i also think that we are programmed to want what we cant have. I would maybe drop hints to mutual friends that you’re seeing someone new too. Lie about it if you have to. If you guys are still friends on social media, use it to your advantage to make her a little jealous and make her miss you and want what she “can’t” have