If it is California-source income, he should be liable, and file a non-resident California return. He can get credit for taxes paid to California on his Georgia resident return - up to the rate Georgia would charge him on the same income.
There have been squabbles about people working from home, and I imagine court cases. Would like to know more about how the court cases turned out. Problem is, these are state court cases and not from the federal system, so the decisions can be inconsistent.