There was no ethical lines crossed. If you have proof that he copied code from your repo then that is illegal. But if he copied the idea, there's nothing ethically wrong with that. The one who crossed an ethical line is you. You can't protect ideas, only actual code.
What is ethically wrong is bullying a former intern with legal threats. You need to learn this, I guess the hard way.
What is ethically wrong is bullying a former intern with legal threats. You need to learn this, I guess the hard way.