Must I do something to atone for my feeling of remorse after having spoken to the manager about an irritable employee's behavior, which caused him to reconsider hiring him?

If your words about someone are truthful, just, and offered as advice, then there is no sin upon you; for the Prophet, peace and blessings be upon him, said: "The one consulted is entrusted." However, if there is exaggeration in it, then you are an oppressor and a betrayer of trust, and you must repent and seek forgiveness. You should also disavow your words to your boss if you can do so without harm. Otherwise, suffice with supplicating that the person be pleased with you.