In this client's case, I'd use the W-2 as prepared.
If the employee is a nonresident and never sets foot in CA, the wages would not be taxed in CA. If the person does make an appearance within the state, that's a different story. CA uses a "source" rule to determine this.
You can read it here under "compensation" : https://www.ftb.ca.gov/individuals/fileRtn/Nonresidents_PartYear_Residents.shtml
Also, here is a very good article that explains it too. Its focus is on the CA auditing businesses with remote employees, but the explanation is there: http://www.sangerlaw.com/Articles/NonresidentsWorkingRemotely.pdf