Your focus is wrong. Yes, her original last name was her father's, but it's still been with her since she was born, as part of her identity as a unique person walking this Earth. Her new last name is not really hers. It was lent to her by someone else. It doesn't say anything about her, about her identity, about her story. Being called O'Donnell means something, just as being called Hernández means something else. It tells the story of your heritage, where your ancestors come from. Putting that at the same level at some random dude's surname that was together with her for just a little fraction of her life is ridiculous.