CITE Sefer Chofetz Chaim chapter 10 halachos 12, 13

Halacha 12: If it’s not so easy to talk about someone who proactively did something against you, then all the more so to tell people how other didn’t do you a favor, like not lending money, or giving tzedaka, or not inviting you into their house as a guest, or even just not being friendly and going over to you with a smile saying hello. In all these cases and the like you cannot repeat them to other people especially if you are condemning an entire community who might be good kosher Jews who believe in Hashem. For example, to say people in New York are cold and not friendly, they are always in a rush and never say hello to strangers even if I am a guest in shul, that would be lashon hara even if it is true. (This example of New York is not to hint at anything specific. I just chose a location with a high concentration of Torah observant Jews all living in the same area.) However, the one exception would be that if you tell people of authority who will be listened to and they are able to speak out in the town and reprimand or inspire them to be nicer then you can tell them because it’s for a positive purpose, assuming your attitude is to try to help and create a positive outcome and you meet all 7 prerequisites. For example, if a person drove through a town with a Young Israel shul and no one went over to him to say hello. If he tells people in the next town what happened would be lashon hara but if he would go to the Young Israel headquarters and tell them and they have the clout to go over to this shul and speak to them to reprimand or inspire them to change their ways, that would be productive and permissible. (The Young Israel example is only given because it’s an easy example to make of an umbrella organization with many Shuls under its name, I was not trying to send any message across.)

Halacha 13: Now, one is absolutely and totally permitted in monetary cases to tell the authorities or people of influence like relatives of the perpetrator or anyone that can help him get his money whether stolen, damaged, or a bad business deal. He can even tell them of a threat so that they can help him avoid losing money or being hurt, as well as if they can help avoid future problems. A person has every right to protect himself and his money and get it back or compensated if owed to him or her. It is not lashon hara to tell the truth to people who can help you. Even in a nonmonetary situation for example to avoid being embarrassed or cursed at you can tell people, even his own family what happened and why you suspect this guy will go after you and scream at you, if you have a convincing story and they can help you avoid being yelled at and embarrassed then you are allowed to tell them the story so that they can convince the person not to confront you or wrongly speak out against you. This is whether to avoid an anticipated argument or to stop an ensuing argument or any embarrassment you think could be caused to yourself in the future. You can go over to them privately and not in a group of at least 3 because Rabbeinu Yona in Shaarei Teshuva (228) only says you need 3 for the sake of the truth to teach others lessons not to do bad things, if otherwise speaking out in public could be misconstrued as flattery or not convincing of being truthful, but in this case your story of a plea for help, even asked in private should be pretty convincing.