Not if there's another man in her life. Don't discount the possibility that she's in love with someone else and that she ran off with him rather than dumping her fiance and cancelling the wedding. She may have also not worn the ring because jewelery can be annoying and uncomfortable when running and working out because of the sweating and chaffing. She was also running at night and didn't want the diamond ring to make her the target of a robbery.
I'd bet if this were ture one of her close friends would be aware of it in some fashion.
Exactly!
If her wedding was a remake of the Zigfield Follies, then her ring probably wasn't a dainty little chip.
I can see leaving it at home for a run.
Didn't she speak to her mother that evening about the last minute preparations for the wedding?