Your constructed hypothetical makes it even worse. If there are 2+ people in this scenario who have good intentions, this should especially never happen. When you sign your name on a paper, you are nonetheless vouching for everything written in it, including the things you didn't personally write. You should absolutely be checking every single reference your co-author included and verifying that it says what your co-author claims it says. This is something you should have been doing completely independent of LLMs existing. This is something you're publishing publicly, something that may be associated with you and your career for the rest of your life, it is insanely negligent to not even read and verify what your co-author is adding.