Criminal Law Questions? Ask a Criminal Lawyer.
Yes. If you are in a place where his prescription is not recognized, you could be charged with possession. Even if he tells the officer that it belongs to him, anyone who can exert control over the area (if it is in the car basically) could theoretically be charged with possession of marijuana.
As a practical matter, if he has prescription, and he admits it is his, most officers would not charge you, but it could definitely happen.
I see. I thought you meant that you would be traveling outside of California. If you were stopped by a State, City, County police officer, nothing would happen. It is legal to possess under the California laws, anywhere in the State. There is no possession penalty under the State laws.