How do we tell him ?

You tell him "stop giving away our secrets!"

And yeah, a lot of people in the comments are running away from the joke, but realistically, to copy+paste code and have it work, you generally have to have a grasp of the code, at least to ask what you want and to paste it and change the variable names, and write the lines to stitch it all together.

Add imposter syndrome on top of that, and it may seem like you don't do anything of use because you copied 3 functions out of a 1k line file.