Lashon haRa, i.e, telling something negative about somebody else, is one of the most serious prohibitions in our Tora and its effects could be devastating. Yet, there are a few instances in which we are allowed or even required to speak up to and reveal certain negative information to prevent someone else's damage, for example, in the world of business (see here ).
In the area of Shiddukhim (=dating) Lashon haRa might cause tremendous and long-lasting damage in the emotional and psychological life of the victims. I know many engagements which were tragically broken, because of Lashon haRa; people --friends, relatives--made negative comments about the bride or the groom, innocently or sometimes deliberately, out of jealousy or resentment.
Still, there might be some 'negative' information about the bride and the groom that we know, and we believe it should be known to the other party. Should we disclose that information?
I will summarize the matter of Lashon haRa and Shiddukhim, with the following two basic rules:
1. If you have first-hand information of a 'serious/objective' matter that affects the bride or groom, you can (or sometimes 'you should') reveal this information to the potential partner. For example, a major issue in the past, like a previous marriage; a life threatening disease, a serious mental problem, etc. However, in other 'subjective' areas--where you apply your judgment more than your knowledge of certain facts, like 'compatibility', character, rumors, etc. you should not interfere. Different personalities might complement each other and make for beautiful marriages.
2. FOR ALL CASES, and before you say one word, PLEASE, always seek the advice of an experienced Rabbi or an expert counselor, to help you assessing if the information you have, it is indeed a 'serious/objective' matter which deserves to be disclosed, or if it pertains to your own subjective judgment, and therefore you need to let the bride, the groom and their families, to decide by themselvesif and when they want to reveal it. A good advisor will also help you decide when and how to reveal the information, as well as verify if this information has been already disclosed, and you did not know about it.