I’m pretty sure that you’re talking about applies when an employee lives in one state full time, but receives a paycheck from a company in a different state. They may live near the state line and commute across the border, or they may be fully remote, but either way, their work is consistently happening in the same location.
In this case, he’s working part time in California, and part time in Washington. The exact laws vary by state… in some states, your tax liability begins on the day you start working there, while others have a certain threshold, but I’m pretty sure he’s crossing whatever threshold there might be. So for the days that he’s working in Washington for a company based in Washington, Washington income taxes apply, and for the days he works from California, California income taxes apply.
Since Washington doesn’t have an income tax, and California does, he will still definitely have to file in California.