..see, this I don't get at all. The ring means something to her, but what? It represents the commitment we made to each other "for better or worse", the commitment she's decided to break. And yet, it's precious enough to save for D5? Why? If the ring means enough to her to save it for sentimental reasons (as opposed to an investment) why not tap into those emotions a little further and make an attempt to reconcile the marriage? These are questions I didn't ask her, and never will.
Some women hold sentimental feelings for things that marked a special time in their life. She may save something from her first date; press the corsage from her first prom; and as harsh as it may sound to you.......her engagement ring. Obviously she has some good memories from the relationship. You are seeing it from the viewpoint of broken vows and a failed M. Maybe your daughter will see it as a time when her parents were happily in love with each other.
I see nothing that odd about it. Your daughter may, or may not, desire to keep the ring. That will be her decision.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!