No, in this case, I would not interpret her income as Massachusetts sourced after her move. According to the language of the directive, if she moved from Massachusetts and it is not due to a pandemic-related circumstance, then it is not Massachusetts sourced income.
The general rule of thumb is that your resident state always gets to tax your income and a nonresident state only taxes it if it is sourced within their state. Massachusetts only considers the income to be MA-sourced if the move was due to a pandemic-related circumstance, otherwise the income is sourced to the state in which the work was performed, her new resident state. In this case, the employer was correct to withhold her new state's taxes and it's a much easier scenario because there will be no double taxation.
She would file the MA Form 1-NR/PY for Massachusetts and indicate she is a part-year resident and allocate only the portion of her income earned while she was a resident of MA from January to mid-March.
She will file a PY form for her other state and claim the remainder of her income there. There should be no double taxation from her wages, as long as she did not work within MA after establishing residency in her new state.
