Reply from Agony Uncle
Is there a written agreement between your friend and your colleague? If there is no written agreement and your friend decided not to rent the room from him then your colleague has no claim for the rental deposit from your friend.
It seems your colleague has no claim against you; even if he is entitled to the sum from your friend (which is not the case) he only has a claim against your colleague. You are not only obliged to reveal details of your friend to your colleague.