Is it permissible for an employee to work with a manager who has committed fornication with more than one girl in the workplace, knowing that he prays and behaves respectfully? And is the employee's remaining alone with the manager at the end of the day considered a private seclusion (khalwa), even if the door is not tightly closed?

It is not permissible to accuse a person of indecency without clear proof, especially given what was mentioned about his righteousness. Nevertheless, it is not permissible to be alone with him, even at work. Unlawful seclusion (khalwa (definition)) is when a man and a woman are in a place where they are safe from the entry of a third party. Prophetic traditions have warned against this, such as his (peace be upon him) saying: "No man is alone with a woman except with a mahram (definition)," and his saying: "Indeed, no man is alone with a woman except that Satan is their third." Therefore, the safest course of action is to leave work in this place. If you remain there, it is not permissible to stay with the manager in the workplace if it is devoid of anyone else besides the two of you. The safety of one's religion is unparalleled, and whoever abandons something for the sake of Allah, Allah will compensate him with something better.