"at the most 10 times last year"

You're stating this as if it is a minor thing. Every detail matters.

"His wife works for a company that resides in California."

It's called Remote Worker. Every State has its own way of handling this. In this case, your taxpayer has income sourced to CA.

"they want a tax on over 47% of the Gross."

It is typically listed on the W-2, or sometimes a second "page" W-2 is provided with State breakdown(s) for remote workers. Did they turn over all of their paperwork?