The teacher spent a good portion of the lesson on how we shouldn't say bad things about someone even if those things are true. One guy in the class raised his hand and proposed an excellent hypothetical that bears discussion.
You know that someone "has a morality problem". [My classmate didn't elaborate. I'm assuming from the tenor of the discussion that what he meant to say was that the individual had pressured a woman he dated into breaking the law of chastity.] Your sister or friend starts dating him. Shouldn't you tell her what you know so that she can be aware and protect herself?The teacher gave lip service to the notion that you should go by the Spirit in deciding what to say. However, she spent the next several minutes talking about how if we share negative information, even if it's true, that it paints a label on someone and prevents him or her from changing. While she didn't come right out and say it, the impression that I was left with was that making it easier for a sinner to avoid embarrassment in the repentance process (assuming they even choose to repent) outweighed protecting the innocent potential victims. I'm sure that isn't what she meant, but it disturbed me nonetheless.
I had an experience on my mission where I had an emotionally and verbally abusive companion. At first, I was reluctant to bring it to the attention of my leaders, because I had had it drilled into me in missionary prep and in the MTC that there's a reason for every companion, you shouldn't complain about them, and if you're having trouble, you should just pray for more charity. I let this go on for several weeks before I told my mission president. He transfered me right away and my companion was sent home a few weeks later.
Part of me I guess believed that since mission presidents are inspired in the setting of companionships, that they know everything there is to know about a situation. Several months later, I was discussing the situation again with my mission president (because I was still distressed over it), and he told me that if he had known what was going on sooner, he would have transferred me immediately. I realized that if we don't volunteer the information we have, then people with the power to do something won't be aware that something needs doing.
I highly doubt that I was this companion's first victim. If one of them had spoken out, I might have been spared the pain I went through. I believe that by speaking out, I spared someone else down the line.
One scripture that speaks to this topic is found in the third chapter of Ecclesiastes. "To everything there is a season, and a time to every purpose under heaven...A time to keep silence and a time to speak." When deciding whether to share negative but true information, I think there are some factors that need to be considered.
First, why are you sharing it? If it's to protect someone, that is fine. If it's because you enjoy passing along juicy tidbits, that's not fine.
Second, who are you sharing it with? Are you telling everyone, or just the people who need to know?
Third, are you violating any confidences by sharing the information? This can get problematic, for example if a lawyer learns something from a client, or the bishop hears something in confession.
If you can pass this test, then it sounds like it's a time to speak. Even Jesus, the perfect embodiment of charity, had some not so positive things to say about some people who were behaving harmfully. He was motivated by love and concern, both for the sinners and the victims. We should feel confident in following His example.