If she now lives in the UK, and the UK was her domicile (her main, primary home) for all of 2023, and she has no MD-source income, then she has no tax obligation to the State of Maryland. In the Personal Info section of TurboTax, she would enter her State of Residence as "Foreign or U.S. Possession", which is the last choice in the state of residence drop-down window.
She must still file a federal tax return. As a U.S. citizen, she is taxed on her worldwide income. She'll be able to claim a credit on her U.S. return for income tax paid to a foreign country.
She should enter her mailing address in TT as the address she wants the IRS to use if they have to send her a letter or a check. Her mailing address and her state of residence do not have to match.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
