My husband's W2 has tax withheld from California, although his work and living location was in Washington State. The employer won't correct the W-2. What should he do?

[fanfare]

If  your husband has not permanently moved to Washington,

his domicile is still CA (I assume that's where you live).

you need to file CA resident and WA non-resident

[fanfare]

I forgot WA doesn't have a state income tax, so there you go, there's nothing to do there.