Written Talaaq

Q: A father of the husband while separated from his wife wrote a divorce letter which the husband signed and gave it to a third party.Some time later the husband decided to destroy the letter. Later on the husband and wife reunited. The wife had no knowledge of the letter and was informed of the letter only after she was reunited with her husband. Many years have passed by. Where does this marriage stand according to Quraan and Sunnah?