I agree with @xmasbaby0's analysis.
He is a resident of CA for tax purposes. Temporarily attending school in Michigan does not change that. Therefore ALL his income is taxable by CA, regardless of where earned.
Since he is a non-resident of Michigan with no Michigan-source income, he has no income tax obligation to the State of Michigan. However, since MI taxes were mistakenly withheld from his pay, in order to obtain a refund he must file a non-resident MI tax return, on which he shows the withholding but declares zero Michigan income.
Washington State has no income tax, so there is nothing to file there.
In the My Info section of TurboTax, he should enter California as his State of Residence. He should indicate that he has other state income from Michigan, so that the program will generate the non-resident MI tax return.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
