Seat rail extension – safety?

Hello together, my parents and I want to make my brother a surprise for his birthday and give him a used car. However, when buying a car, there will be difficulties, because my brother is very big and therefore has very long legs. He suffers very much from his height. I have heard from a friend that you may be able to extend the seat rails so that the driver then has more space. He said that I should inquire because it may be too Prob Can you help me? What should you look for with seat rail extensions or are there even better solutions to create more space for the driver? I would be very grateful for your help. Best regards, Halfmoon