What is the ruling concerning someone who knows that a certain individual is untrustworthy and has himself been the first victim of his untrustworthiness, yet conceals this fact from others when they consult him, especially when their financial rights may be at risk because they have financial claims against that individual?

A Muslim is obligated to offer sincere advice (nasihah) to others, as a person who is consulted is entrusted with a responsibility. Tamim al-Dari (may Allah be pleased with him) narrated that the Prophet (peace and blessings be upon him) said, “Religion is sincere counsel.”
We [the Companions] said, “To whom?”
He said, “To Allah, His Book, His Messenger, the leaders of the Muslims, and their common folk” (recorded by Muslim).

Based on this, whoever is consulted about an individual whose treachery he knows of, yet conceals it from those whose rights may be jeopardized if they deal with him, shares in the sin by neglecting to offer others the sincere advice he is obligated to provide.

 

And Allah Almighty knows best.