Re-register the car, where?

Hey, I have a question about the registration and insurance. My friend buys a car next week, his insurance man told him that the best thing about the insurance is to register the car as a second car with his mother (VHV). His mother lives in town A, but he lives in city B, far away, he buys the car in his city. The insurance doesn’t allow UN and holder to differ from each other. Now the question, how does it work most easily with the re-registration? Is that clear, we have to have the car without a license plate on a private car, send the license plate to his mother, who reports in her city around with registration plate for city A, sends it back and we build it? I find that very complicated. Is there any other possibility? Greetings and thanks lemo