He would need to file in his resident state. You do not provide enough information to determine this decisively, but see Guidelines for Determining Resident Status (FTB Publication 1031). He is most likely still a California resident so he would have to file a California income tax return.
Because he worked in Wisconsin, he would have to file a nonresident WI tax return. Because teleworkers residing in California working for a company in another state due to COVID-19 will not be taxed in California, all of his income would be taxed by Wisconsin.
See also How do you file taxes for 2 different states? - TurboTax.
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