It's just that I'm so picky with his designs, and if he does something wrong, that usually means the design is ruined. He just revamped a Chesire Cat fakemon, and while it's nearly as chubby (Cute!) as the original, it's tail is smaller, it's hands are stubbier, it's feet are... pretty much the same as the first version. It's arms look a slight less powerful, and it's face is a slight less sinister IMO(The last one's debatable). My biggest problem is the smaller tail, it gives the design less density. Me, dense monsters are my favorite.
It doesn't help matters that this guy doesn't take criticism well. I have a feeling he's going to play the "You do better then," card or something like that. It's not a matter of who's better at drawing, it's just a few suggestions.