He would file a part-year resident return for CA, and allocate his WA state earnings to WA.
In the Personal Info interview of TurboTax, he should indicate he was a resident of WA on 12/31/2021 and that he "lived in anther state 2021", i.e. CA. The program will identify him as a part-year resident of both states and prompt hm to prepare a part-year resident return for CA (WA has no income tax).
In the CA state tax interview of TurboTax, there is a screen where he can "allocate" his 2020 income to those states where he resided and worked. Even if the employer reported him as receiving CA wages and withheld CA taxes, he can correct that in the CA non-resident return interview.
