The California Department of Motor Vehicles (DMV) site states that CA drivers and vehicle owners are required to carry minimum monetary limits of liability auto insurance. So this statement seems to say that even if you do not own a vehicle that as a driver you need to carry car insurance while you have a driver's license in CA.

Your friend can check with their insurance company to see if you would be covered under their policy when driving their car. If you are going to drive the car on a regular basis you might need to added as either a driver or occasional driver. Typically, you insurance coverage follows a car and not always the driver. So, when you are driving their car, their insurance should cover you. It is important to check with their insurance company because many policies have exclusions for these cases. For example, you can have an exclusion for any driver under the age of 25 on a policy.