They want to be an employee of a company in California, while living and working in Canada. This sounds like it's going to be complicated, and a blanket answer of "you don't pay US taxes" seems unlikely to be correct. They're also going to face issues like on day 1 when they walk into the office and they're going to have to go through the whole I-9 thing (
https://www.uscis.gov/i-9-central) - waving hands and saying I'm going to live in Canada seems like it's not a solution.