Dear All,
Thank you for the advice. The attitude of this girl is very strange. She received a warning letter (as per my boss's request), and since then, she has been in rival mode. She even fought for the warning letter, claiming it was sent without any relevance, although she deserved it. Now that she has resigned, she is in revenge mode. She has been extremely rude and lacking manners. She refuses to accept any policy and instead blames me. Here are some of her words: "But since you're being unfair to an employee in every way, I really don't wish to discuss this further with you. It's not my fault if your documentation is improper."
She has also shouted at me without any reason during a meeting with the CEO. Essentially, she is doing as she pleases because she is leaving the organization.