Under the old law of domestic relations, when a marriage proposal is accepted, a contract is formed. In fact, you used to be able to sue for breach of promise to marry. When the contract is breached, the person who breaches the contract is not entitled to the ring. In other words, if the proposer breaks it off, the other person gets to keep the ring; if the person accepting the proposal breaks it off, the proposer gets the ring back.
I wish this former couple the very best of luck in resolving their case for less than the replacement cost of the ring at issue.