OP's description sounds like they are working alone on this project and that there's no peer review. Which sounds like a weird thing to do for a junior dev that just got out of college.
Yeah that definitely falls on "you probably need better process in the company as a whole"
If they really slipped a deadline because a junior developer wrote code with design issues even when it was pointed out by his manager, it's the managers problem. The way to deal with that is tell the developer "this is the new peer review process" and just stomach the delay.
