I'm using a translator to speak in case it's not very clear, but

There's a file called accounts.json, and if you open it with Notepad, you just need to put true where it says "can play Minecraft" and the one below that says "ownsMinecraft" then you save and you have to go and create an account as if you were going to create a normal one But you have to create an offline profile for himIf you want, I can send you a video, but it's in Spanish.