there are many factors that come to mind.. if it was given on a holiday (vday, christmas, birthday, hanukkah) then no, if it was given on any ordinary day yes or if it is a family ring then yes.
my friend actually went to court for this.. he proposed on V-day.. she broke the egagement off and kept the ring, the judge ruled in her favor (male judge) and said it is considered a gift. she got to keep it. he did give a word of advice to the men in the court room, dont do it on a holiday. lol. none of my male friends after that did it on a holiday
I think if it is a family heirloom it should be returned, bc she is no longer part of the family and it is NOT her property. I also think it depends on the terms of the breakup. If he cheated she can keep it, if it was mutual and just fell out of love, I think the respectable thing to do is to give it back.
I think it should be returned. It was given to her as a gift to be married. If the marriage isn't happening, the gift would never have been bought otherwise, so I honestly think she should give it back.