The prohibition of lashon hara forbids speaking harmfully about another Jew, even when it is true. The obligation of ahavas Yisroel requires a person not to say about someone else what one wouldn’t want said about himself.
However, it is only lashon hara if it is repeated for no positive purpose. If the information is shared for a reasonable purpose there is no prohibition of lashon hara, provided that: (a) One is certain that the facts are true. If one heard it from a reliable source he should say that this is what he heard (a rumor should not repeated at all). (b) The facts are stated, with no exaggerate. (c) One’s intention must be to help and prevent potential harm. If one is also interested in besmirching the prospect, he may not share the information and should recommend someone else who they can speak to.
One may only share information that will bring benefit. However, if the inquirer trusts the judgment of the responder, one should only say that it’s not a good shidduch without sharing the details. Likewise, if the shidduch was already finalized, one may not warn them of a flaw unless it justifies canceling the shidduch.
To avoid misleading the inquirer (in either direction), one should not offer general descriptions (“he’s a chassidishe bochur” “she dresses tznius”) but should offer concrete examples. Also, since certain flaws may be suitable for the other party (“he’s absentminded” “she’s messy”), one should first inquire about what they are looking for before sharing irrelevant negative information.
The one who is researching the shidduch should not accept the information to be definitely true, but rather as a possible concern. He should state clearly that he is calling for the purpose of a shidduch so that the one answering is not transgressing lashon hara. One should not approach an enemy of the bochur or girl, since even true information will be shared not solely for the purpose of benefiting the parties, thus causing that person to transgress.
Yet in certain situations one is obligated to share the information as the Torah says "lo saamod al dam rei'echa," that one is obligated to save another person from a calamity whenever possible. Therefore, if one knows that a shidduch will bring certain tragic results and one can influence them, he is obligated to speak up. One who remains silent commits an aveira.
If one can stop the shidduch without sharing the specific information, he must do so. If by sharing the information it will cause himself harm, some hold that he is not obligated to jeopardize himself. However, he must find a way to relay the information.
Regarding helping relatives poskim point out that the Navi says "mibsorcha al tisalom," that there is an obligation to show added care for one's own family (some add close friends as well).Therefore when a close relative asks about a shidduch one should share all relevant information even if they don't ask (just like one would offer unsolicited advice to family members about other matters). On the same premise, a person is not obligated to reveal light flaws about a relative or close friend if he wasn't asked directly about that detail.
If a shidduch prospect has a serious health issue which will complicate life but will not ruin the marriage (e.g. diabetes), one is obligated to share that information if called, but one need not reach out to tell them. If it is a minor issue (e.g. food allergies, skin ailment) one need not mention anything unless one is asked about health issues.
Since individual situations vary greatly, it is advisable that one consult a Rov with the specific shaliah.
