What should a Muslim employee do if they witness injustice at work, and is informing superiors about the conduct of non-compliant employees considered discord (fitna) or is it a duty, and what is the optimal solution to this problem?

What was mentioned about the two colleagues is reprehensible and must be changed by advising them and instilling the fear of Allah in them. If they repent, then praise be to Allah. If they do not desist, their matter should be referred to the authorities to clear one's conscience. If the change can be achieved without mentioning names, that is better.