I’d say if the guy breaks the engagement, it’s hers.
But if she breaks the engagement, it goes right back to the guy.
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Legally a ring is consideration to bind a contract and returns to the owner if the contract is broken. I think there have been a number of court rulings to this effect.
I know that has been a ruling and I think it is wrong—perhaps going back to the days when women didn’t hold equal property rights?