He probably clicked on a fake Discord invite link for some false free gifts or a view more link. Unless he had backup verification set or pays a ransom, the hacker likely will make it impossible for him to reobtain the original account so they can keep spreading the message and he'll need a new one. I'm not sure if a ban was needed if a kick is enough, but I guess it's a very public server to be able to rejoin. It's a real shame.