My friend was born, in Mexico, during the 1950s. She did not have dual citizenship. She was Mexican until she naturalized as a U.S. citizen in the 1970s.
You must think I don’t know the law. With regard to persons born overseas to U.S. citizen parents, the Congress has the power to determine whether they are citizens at birth and to determine what they might have to do to not lose that citizenship. This was spelled out in Rogers v Bellei, concerning a person who was born overseas of one U.S. citizen parent and was a citizen at birth, but who lost his citizenship for not fulfilling the requirements. The Congress in its wisdom has changed these requirements over the years, so it would require a lot of work on my part to speak with specificity as to the case you mention. Nevertheless, a friend of mine, now retired, would have been happy to represent a person such as you described in a claim to be a U.S. citizen.
Does it surprise you that the Donald is now questioning Marco Rubio’s qualification as a natural born citizen?