How should one deal with an employee who spies on colleagues and divulges their secrets to managers? Is it permissible to pray behind him, offer him condolences in his misfortunes, and offer condolences upon his death?

Backbiting (namimah (definition)) is forbidden, and whoever is known for it with the intention of causing corruption is a fasiq (transgressor). Prayer performed behind a fasiq is valid. It is permissible to congratulate a backbiter and offer condolences to him. The preferable action is to do what serves the best interest: if he would be deterred by ostracization, then ostracize him; otherwise, advise and admonish him, keeping in mind that some نقل الكلام (conveying of speech) may be permissible.