If she is a resident of MO, then all her income is taxable by MO, regardless of where she earns it.
As to whether or not she must file a MO tax return, see "Am I required to File a Missouri Return?" in this online reference:
https://dor.mo.gov/faq/taxation/individual/general.html
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
