As long as he does not make any money and it is not about a current gen game, its not as bad as you might think. He does not directly use assets from Nintendo, as everything is self created. The only thing in the way is, that he is using the IP (Intellectual Property) and names. But you know what, there are thousands of fan made games like that - ton of them even using assets directly ripped off Nintendo games, and they are not sued. People do this kind of thing since decades.