That’s not just a process error. At some point you just have to feed back to the right person that someone isn’t up to the task.
As an example there was a case where some buttons needed a special highlight based on some flag, something that could be done in 4-5 lines of code or so (this was in Unreal Engine, the UI is drawn each frame), but the PR was adding a value indicating if the button would need to be highlighted when the button was created and this value was passed around all over the place in the up to the point where the data that was used to create the UI with the buttons would be. And since the UI could change that flag, the code also recreated the UI whenever the flag changed. And because the UI had state such as scrolling, selected items, etc, whenever the UI was recreated, it saved the current state and restored it after it was recreated (together with adding storage for the state). Ultimately, it worked, but it was too much code for what it needed to do.
The kicker was that the modifications to pass around the value for the flag's state wasn't even necessary (even ignoring the fact that the flag could have been checked directly during drawing) because a struct with configuration settings was already passed through the same paths and the value could have been added to said struct. Not that it would have saved the need to save/restore the UI state though.
This is outrageous regardless of AI. Clearly there are process and technical barriers that failed in order to even make this possible. How does one commit a huge chunk of new code to an approved PR and not trigger a re-review?
But more importantly, in what world does a human think it is okay to be sneaky like this? Being able to communicate and trust one another is essential to the large scale collaboration we participate in as professional engineers. Violating that trust erodes all ability to maintain an effective team.