i dont thik dual citizensip is legally possible. the oath naturalized citizens take to be an american citizen clearly states they renounce all other citizenships.
Unless she officially renounced her British citizenship, she will still be considered a British Citizen in Britain with dual nationality. I think this applies with most countries. Simply accepting the citizenship of another country is not enough to renounce your original citizenship, regardless of what the legal position in the new country might be.