Thousands of people/students head off to another state to work during the summer. National parks, beaches, amusement parks, internships...etc. They don't become residents of that other state because they don't intend to stay there to become permanent residents of that other state...and they don't do the things that show they are truly changing their domicile...like change their Driver's License, register to vote, re-register their car...etc.
As such, for the situation you describe she was working as a non-resident in WA, and all her income there would be NC taxable (Assuming you were both NC residents to begin with).
When a person works the summer in another state that has an income tax, and their income is taxed there, then the NC tax return would still include and calculate a tax on that income, but, NC would then allow a credit to be applied for the taxes paid to that other state....unfortunately, WA does not have an income tax...so no credit in this case.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
