Sefer Chofetz Chaim hilchos rechilus chapter 9 footnote 9

This 5th condition that you cannot harm the would-be perpetrator any more than what he deserves is a very tricky condition which the Chofetz Chaim himself is very hesitant in what to do but bottom line one has to divulge information in a fashion which will be a balance for both parties. That the wouks be victim can take proper precautions and the would-be threat won’t be overly damaged than what he or she deserves. For example, If Reuvain wants to hire Shimon for a job and Levi knows Shimon stole something at one point. Levi has to tell Reuvain the information he knows even if Shimon regrets what he did and wouldn’t do it again, but Reuvain has a right to be concerned and investigate the matter. However, if Levi can determine that by telling Reuvain about Shimon the word will get out and Shimon will be run out of town in shame then Levi really shouldn’t tell Reuvain the information. In this case as opposed to lashon hara, one does not need to have seen the stealing happen first hand, or even know for sure that it happened, as long as there is rumors you are allowed to tell a would be victim to be cautious himself just as you yourself can just be cautious with this information as long as you know he will only take proper precautions to quietly investigate the matter and make proper decisions to just protect himself from any harm. Just as a side note, even though there is a specific mitzvah of “lo sa’amod al dam re’echa” don’t stand by when your fellow Jew is in danger that is only when you definitely know of the potential danger, and you can help. But if it is just a rumor then it’s a good thing, a general mitzvah, to be concerned about and take proper precautions. So Gedalia ben Achikam, the mayor of Beitar that refused to accept lashon hara and wasn’t even cautious and that is why Beitar was destroyed, and we have Tzom Gedalia, chas vishalom, a tzadik like him transgressed a blatant mitzvah, rather he was being too Stringent of not even being cautious about rumors that he heard that he might be killed by someone.