I don't think so. The ring was contingent upon her following through with the agreement.
No guy wants to sink big bucks into a ring, just to have his fiance reject him, and then keep the ring as an expensive trophy. That's like adding insult to injury.
I mean, what's the point? Wasn't the ring a symbol of the love they had for each other and their intentions to marry one another? If she nixes the deal and puts those intentions asunder, she should graciously give the symbol of their love back to him. It's the right thing to do. And, it may also help him during the healing process, because if she keeps it, he may feel there is still a chance.