My friend sucks at LoL

Hi. I have 2 friends that play for the same amount of time, 1 of them is good and the other 1 is terrible at this game!!! Seriously he just sucks. He has like a 30%win rate and that wins are from when he plays with me and my other friends. But the other 1 that is good at the game plays OK. When me and my friends are talking on skype and we want to play League and the bad guy is there, we just go for a ranked without him and hope that he dont notice us. He plays LoL for like almost 3 years and he has skill of a level 30 unranked. I would like to help him improve but idk how. Just to prove how bad he is i will tell you a short story about a gank that he has done to me. Well... He thinks he is a good jungler cuz when we play with him we say to him to go jungle ( cuz then he do not feed). And one time he was doing a counter gank on my lane and their jungler was like 50 hp with ignite on him, and since we was dead with ignite i went to the midlaner that was at the same hp as me and because i thought my friend was there. 3 seconds later i died and i asked him why he wasnt there. Guess what... his response was that he was chasing the enemy jungler when he was already dead. Since then i never played with him again. I hope u can help me teaching him the game because idk how to do it myself. And sorry for the bad english.