if the car is being stored it can still rust. U dont need salt to rust something. Its chemestry. All u need is water and bare metal, bam u have rust. Salt is just an electrolyte which speeds up the process of reduction and oxidation. Its a redox reaction.
There for if he has the rust module on the car, he is just protecting it whether its inside or not. The ideal place for a car would be in a de humidifed roomm so there is no moisture in the air