Why is Corey so Bad at League of Legends

Alright, i have a friend named Corey who regularly plays League of Legends. The problem is, he is complete and utter garbage. He wants to become an Azir main but cannot hit his ultimate or simply have a good KDA in ANY games, he's really trash. My friends and I decided we should make him learn the core game mechanics of a midlaner by making him play Annie, as he cannot do simple things such as CSing. He sucks ass. He can't hit his annie ultimate, watch his stun or even CS with Annie q. Is there any way he can stop? Thank you for your time <3.
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