How can I differentiate between giving advice and backbiting when warning a colleague at work about another person, especially with the prevalence of speaking ill of others at work, even of managers?

The basic principle is that a Muslim mentioning his Muslim brother in his absence with ill is considered forbidden backbiting if it is truthful, otherwise it is slander, and both are forbidden. However, backbiting is permissible in certain situations, including advising Muslims and warning against evil and the people of innovation and vices. It is permissible to mention the transgressor to warn against him, and it may even be obligatory.

If the colleagues' conversation is in the nature of advice and warning against negligence, treachery, or neglect that harms the interest of the work, then it is permissible, provided that the intention is to reform and do good, and that the motive is not envy and competition, and that it is done in the manner of advice and seeking the face of God, and that it is limited to the extent necessary to achieve the advice and reminder, as the basic principle regarding honor is inviolability.