In California, "joyriding" only applies to boats and bicycles.
It sounds like the most likely charges are Penal Code 32, accessory after the fact (helping someone avoid arrest, knowing they committed a felony) and Vehicle Code 10851, driving a vehicle, knowing it is stolen.
Both are "wobblers" that can be charged as misdemeanors, with up to a year in jail, or felonies, with a maximum state prison term of three years.
To be guilty of an accessory after the fact, your boyfriend would have had to have known that his relative had committed the robbery before he moved the car, number 1 and #2 that he moved the car in order to assist his relative in getting away with the crime. Tell him to get a good lawyer as he might have a potential defense.