If his code is sub-par, the professional response would be to make comments on his commits and/or otherwise try to provide guidance/mentorship. Not completely

If his code is sub-par, the professional response would be to make comments on his commits and/or otherwise try to provide guidance/mentorship. Not completely strip out his code behind his back and replace it. The problem here actually sounds like this coworker is used to working alone and doesn't know how to work on a team.