Nope. She’s British now.
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Did she renounce her citizenship? As I understand it you have to specifically renounce your citizenship when you take another otherwise you are considered a dual national. A lot of Americans who take citizenship of another renounce American citizenship for tax reasons as unlike most citizenships, you are required to pay US tax even if you live and work in another country. Boris Johnson, our most likely next PM was a dual national until 3 years ago (born in NYC) and then gave it up. Presumably he did it because they conflicted with his political ambitions, its kind of awkward aspiring to be PM whilst not being a full and exclusive British citizen...