My friend is a commercial truck driver. In August he was driving his employers truck when he pulled into the scales. He was given a fix it ticket for something having to do with a rear torque arm bushing. When he returned to the shop he gave his supervisor the ticket. His supervisor told him that they would take care of it. Last week my friends insurance company notified him that his license had been suspended because the ticket had not been taken care of. What are his options? Surely he can't be held responsible for fixing a vehicle that isn't his. We are in California

Unfortunately the burden is on him until he gets his employer to handle it.
(This happened to someone I know, who was dragged out of bed in the middle of the night and thrown in jail for not handling a ticket for driving his employer's truck with an expired inspection.)
He needs to threaten the boss with reporting to the DOT and the DMV and the Dept of Labor and effectively put him out of business if every part of the situation isn't resolved in full. And sue him, personally, in civil court, for losses and legal fees.
He has a right to be angry, and he needs to follow through on his promises if the employer does nothing.