Shemiras Halashon Halachos
Illustrations paragraph 4 If Levi sees that Reuven wants to make a marriage arrangement with Shimon and knows that the bridegroom has serious flaws which Reuven does not know of, which would lead to Reuven not making the marriage arrangement, Levi must inform Reuven. But there are many conditions which we will explain later. Above all, Levi must be very careful before doing this and he should not compare leniencies of one case to another case. Before we begin to explain this law in detail, we will explain common stumbling blocks relevant to it. Even if it seems there is no point writing them as they are simple, we will explain them because of the great misconduct that is prevalent in this matter.
Illustrations paragraph 5 One must be very careful concerning undesirable character traits. People often denigrate someone for things that don't exist. For example, if the bridegroom is a straight person who does not understand people's cunning or does not joke around like other youngsters, immediately, he is publicized as a fool and simpleton to the extent that the marriage arrangement is sometimes annulled. There are many examples of this.
Such people are not merely speakers of lashon hara. The Torah calls them motzi shem ra (slanderers) because they say untruths. Even if it was true, one would need to fulfill all the required conditions. Therefore, to denigrate one side of the marriage agreement because of its ancestor's deeds is also lashon hara.