Translation:Her one wish was to see her son again.
Without more context, you can't know. I'm not on the Spanish team anymore (and don't remember what the responses were), but from the top of this post, it looks like the primary response is "her one wish was to see her son again." So the system should accept "her," unless there's something funky about the way the sentences were entered. Are you sure there wasn't another mistake somewhere in your response? (If there was, Duo would have suggested whatever it thought you were trying to type, and depending on the mistake, it might've guessed "his.")