When it harms the users by crashing their game. Like Greg did. The only "harm" from mDiyo's recipe alteration of Greg's recipe alteration is to Greg's "vision", which was clearly interfering with mDiyo's. And once again, mDiyo did *exactly what Greg does with his recipe changes*. Why is it ok for Greg to do, and not mDiyo? The harm from Greg's code affected every single user with both mods installed. It's pretty simple to see which one is worse.
Even if you want to argue that mDiyo "started it" (as opposed to changing the very first recipe in the game "starting it"...), Greg's retaliation is clearly worse overall.
Offtopic: Seriously, sometimes I wonder if Greg and his supporters are secretly just massive trolls who love to see people annoyed. I cannot fathom people defending willingly crashing the game for every user as a response to hypocritical spat about someone changing his recipes, especially considering that is one of the main functions of Gregtech