Some more shidduchim and shmiras halashon points: (source: shidduch addendum to the Chofetz Chaim: A Daily Companion)
– If you have negative information to relate about a prosepctive shidduch, yet you know that the person inquiring will not heed your advice, then do not disclose that info.
– If you decide not to continue with a shidduch based on info you heard, you may not repeat this info to anyone else, even to the shadchan. An exception to this – “unless there is a clear purpose”, i.e. “in order to prevent another person from falling into a bad situation”.