@sakonidas said " Therefore, is she only subject to NY tax for those 12 days."
No, that's not how it works with NY. Even 1 day in NY, makes her subject to NY tax on her full year salary.
Here's a link to New York's memorandum on its "convenience of the employer" tax doctrine regarding non-resident telecommuters: https://www.tax.ny.gov/pdf/memos/income/m06_5i.pdf
