Looking for Advice...

I have a friend I met through roleplaying games, and when we discovered we both played League, we were like "cool, let's play together". And that's all well and good, but there's one major problem.

He's really bad at League of Legends.

He's the kind of guy who plays champs like and as supports, then stands in front of the minion wave to poke the enemy team, then wonders why, as the game goes on, he's died so much and tilts off the face of the earth because of it. It's frustrating because I know he can do better. I've seen him improve on his positioning and sense of when to engage, and I've seen him play mid lane with his favored champions and do really well. But the only guy who had the patience to deal with him hasn't been around due to irl reasons, and without him, my friend's skills just decayed, and he refuses to play mid anymore because he doesn't want to face the challenge that is laning alone.

It's gotten to the point that I don't want to play with my friend anymore because playing with him is a frustrating experience, especially when he "pokes" by sending W through the minion wave to try and poke the enemy laner, and takes half my CS in doing so. I've been ignoring his requests to play with him (which are numerous, regardless of whether or not I respond), and when I play and he's not around, I feel like I've made some actual progress in terms of improving my skill and raising my rank.

My question is, how do I resolve this situation in a way that's not going to make me the bad guy, but lets me play alone when I want to without being expected to play with this guy whenever we're both online at the same time?