Is what the questioner did, when he informed the branch manager that the secretary was sitting with another branch manager known for his illicit relationships, considered slandering chaste women (qadhf al-muhsanat)?

You are right to condemn the girl's private meeting with the man. Being alone together for a man and a woman who are not mahram is an invitation to temptation and a cause for the spread of indecency. The Prophet, peace and blessings be upon him, warned against it by saying: "No one of you should be alone with a woman, for Satan will be the third one between them." And your informing that he was alone with her is not considered slander.