Sefer Chofetz Chaim hilchos rechilus appendix, halacha 4, 5

In terms of shidduchim, if a boy is being set up with a girl and you know of a major character flaw or any other major issue of the boy or vice versa then you have to tell the other side if they don’t know. Halacha 6 will define what’s a major flaw and we’ll discuss all the many complex parameters of how to reveal to the perspective shidduch. However unfortunately people are very quick to tell others about a perspective shidduch information that does not need to be shared, maybe they have good intentions because they don’t want anyone to be hurt if the shidduch goes through but if the information isn’t a serious problem, then it need not be shared and ruin a person’s identity. For example, if a guy is simple minded and people take advantage of him, or if he is serious and does not joke around too much. To make the guy look like a fool and ruin him is uncalled for, and there is no reason to tell the other side of frivolous flaws whether it is true or in true. People who go around peddling character flaws which are insignificant should not be listened to and people who listen, even if they at first are innocent and just listening nervously, will eventually stand by and listen and if they continue they will sit down and listen attentively and join into the mocking of others. The baalei lashon hara that speak it are causing the public to sin and the best thing to do is not to confront them with rebuke but rather just to stay away. Whether they are making fun of the boy or girl themselves or their family it’s all the same. Even if the flaws might be true, to meet all the rules in order to be allowed to say it is pretty much impossible in these situations. There are situations which we will elaborate on, b’ezras Hashem, that one could say something before dating starts but once they are going out and don’t see the issues with each other it is no point bringing up and highlighting those type of issues.