Yes, because he earned the income in California, he must file a nonresident tax return to pay state income tax. (California has a pretty low threshold). Because Arizona is a reverse credit state, you will want to complete the Arizona return first, then he can apply the credit for taxes paid to another state to his California tax return. NOTE: if you are not sure if you need to file the California, you can run through it in TurboTax and delete it if there is NO tax liability.