His company is following NC tax law. NC can tax non-residents on "NC-source" income. Income earned from work physically carried out within NC is NC-source income. He must file a non-resident NC tax return if his income exceeds NC's filing threshold, which is shown here:
https://www.ncdor.gov/taxes-forms/individual-income-tax/individual-income-filing-requirements#Filing
Note that the NC filing threshold amounts are based on federal gross income, not just on NC gross income.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
