Which cars are covered depends on the roadside assistance provider. AAA services are available to members and not specific to any vehicle. As long as members pays their dues and have their membership card on hand, they can use AAA for someone else's car.
Generally, auto insurance plans cover the car itself, not the driver, so people can't use their roadside assistance for another. However, details vary between providers, plans and local laws. Drivers should consult with their insurance providers to see whose cars are covered.