I would have felt more threatened by him keeping the ring. The fact that he could give it back to her just like that, tells me that he has no feelings for it and harbours no sentiment for it or their time together. If the mom wants it, let her have it. Technically, it was a gift for her when he proposed, and once you marry and divorce, the husband has no claim on the ring (irrespective of value) because it is seen as a gift, it is not even included in the assets to be divided. What she wants to do with it, is up to her. Sometimes we get silly feelings, once we work out what really bothers us (obviously it is not about the ring) the feeling can be handled. Don't leave it though. Try and figure out what is really bothering you about this.