SHAFAQNA – The Grand Ayatollah Sistani answered a question about disclosing secret to prevent sin.
Question: A father asks the friend of his child to watch his behaviour and after a while ask the friend about his child’s behaviour. Is it allowed for the friend who does not like the father to find out about some characteristics of his own child, to disclose to the father those characteristics?
The Grand Ayatollah Sistani: It is not allowed unless at a time when that characteristic is a sin and preventing that sin which is Wajeb cannot be done except by disclosing the secret.
Source: PERSIAN SHAFAQNA