well, two things. Version Control and something working but similar.

Means, you teach them how to get back to the working start state. Then you check out the repo: https://github.com/topics/2d-platformer-game

And you help him to modify it. Add and expand it. Modding is the way to get into gamedev, without the demotivating problem of having nothign to play and long workhours without reward at the start.