If the guy doesn't post it on the forums and Badspot never hears nor sees of it, there's no way that he could be banned or have his key banned. If it's simply a private add-on that he enjoys by himself, or with a few friends who won't spread it around.
It's like herpes. You can have herpes, and even share it with a few people, but once you start giving it to everyone, it's best time someone shut you down.