An engagement ring is given as a gift in contemplation of marriage. If the marriage does not take place, the ring should be returned. There is a lot of case law about this, but most of the controversy has to do with return of the ring after the marriage took place.
I agree w/ the others. Notwithstanding the timing of the gift, an engagement gift is typically in contemplation of marriage and if she broke off the engagement, she is bound to return the gift.