I would not file a part year CA return if he never lived in CA.  Or file part year if he did live in CA for part of the year. I can't tell. " was transferred to Nevada" You don't say from where.

The income earned during the 3 weeks in CA is probably taxable by CA - on a CA nonresident return.

Your client needs to get correct paperwork filed with the payroll dept of his employer to show he is not a CA resident.