Destructive Git Records?

I'm starting to learn Git, which is great so far. I just got confused in one area, it may just be a misunderstanding.

Suppose John is in the john directory and Bob is in the bob directory.

If John’s code is not the last item we want for the john directory, and Bob pulls from the dev branch, Bob will get all of John’s “bad” code, right?

So, Bob applied a simple fix to the "bob" directory and clicked on the production branch. Is it not bad that the John code in the "john" directory enters the production branch via Bob?

Is this really happening? How to avoid this?

Thanks everyone!