Mama said:
"If you have nothing good to say, say nothing at all."
That pretty much sums up the teachings against detraction, which is saying something which is true that harms the reputation of another, and calumny, which is telling a lie that harms the reputation of another.
The Catechism has this to say:
2477 Respect for the reputation of persons forbids every attitude and word likely to cause them unjust injury. He becomes guilty:
— of rash judgment who, even tacitly, assumes as true, without sufficient foundation, the moral fault of a neighbor;
— of detraction who, without objectively valid reason, discloses another’s faults and failings to persons who did not know them;
— of calumny who, by remarks contrary to the truth, harms the reputation of others and gives occasion for false judgments concerning them.
As the definition in the previous post indicates, there may be exceptions when “there is proportionate good involved,” or as the Catechism teaches, when there is “objectively valid reason” to disclose such a truth. If, in your situation, you think there may be good reason to disclose something against someone, perhaps you could elaborate for us, or it might be better to seek the advice of a priest or spiritual director.