It sounds like your employee may be working for you under more of a contract labor scenario, in which case you don't have to deal with payroll taxes etc., just issue a 1099 at year end. I would make the switch to contract labor upon the employee moving. Because the employee is both living in and working out of WA, you would withhold according to WA tax law and schedules, which is good, because WA has no personal or corporate income tax:
Here is a link to WA workers' comp requirements:
Here are the employments that are excluded from WA workers' comp:
This stuff is black and white:
Even though your corporation was created in CA, you can end up owing income/payroll/property and other taxes where you establish nexus with different taxing authorities.
To be blunt, if you're going to have an employee working out of WA, you're going to have to adhere to WA laws. I did tell you that WA has no income tax, so that is good for you.
I'm sorry there isn't a correct answer that is going to tell you what you want to hear, but that is the answer. Thank you again for your question.