[mayoi]

> He submits PRs that have lots of weird merging issues [1] that are too complicated to unpack and I end up creating new branches to cherry pick his contributions manually.

You could start by not doing this and instead working your way together to fixing the problems because that's literally what PR's were made for, even more so on GitHub where unlike in email, you can discuss it in almost real time. It looks like you have as much to learn as him, because even if you have to make 500 commits to fix all issues, there's this git feature called commit squashing that helps you clean things up even after the fact, and branches, especially personal ones, can easily have their whole history overwritten without fear as many times as you need it.