The Teshuvas Chelkas Yaakov Vol 3, Siman 136 cites the opinions of the Chofetz Chaim that there are three important areas that need to be disclosed.
1) an internal illness.
2) Heretical illness
3) if the person is habitually engaged in immodestry.
The Chofetz Chaim expands on this by saying that each of these is only considerd on as serious level. If a person is often very weak then he is not classified as being ill. Similarly, a heretic is one who airs heretical views, not just that he’s ignorant in Torah.
Someone who needs a special diet, but it doesen’t compromise the potential longetivity of his life, is not considered to be ill in the respect that his condition must be revealed.
Rav Moshe Feinstein in Igros Moshe writes that if you know that a woman can’t have children but perhaps not as many as other women because she is weak then you don’t have to disclose it.
The Chofetz Chaim writes that even when there is an obligation to disclose anything then one needs to satisfy four criteria before disclosing it. First, one must have gained the knowledge first hand, not that one heard it by hearsay. Secondly, the information has to be said accurately, and not in an exaggerated manner. Thirdly, one can only relay the information if it’s going to be heard and accepted. If the person has already made up his mind and is not prepared to withdraw his objections, then there is no point in saying anything. Finally, one has to say it ??????, solely for the benefit of others, not to harm the party about whom the information is said.
What happens if one of these conditions is missing? Many times one may have heard something from a reliable source but one doesn’t posses first-hand information. The Gemora tells us that if we hear something from a reliable source then ????? ???? ???? we can show a degree of concern but may not accept is as fact. What, therefore, is my obligation towards disclosure?
The Ksav Sofer writes that there exists no obligation to disclose it but nevertheless you are allowed to disclose it.
The Chofetz Chaim writes that the heter from the Ksav Sofer to repeat information that you heard from others is only true if the person to whom you are forwarding the information will also accept it to the point of ????? ???? ???? but not as a matter of fact. In other words they will investigate it further and judge properly. The Chofetz Chaim adds another rule; if there is a issue that needs to be disclosed to no party concerning an act of deceit from the other, then it is only permissible to tell them if they themselves are not fooling the other party. However if none of the parties are deceiving each other then it’s a good shidduch and one can let them go on their merry way.
The Chofetz Chaim adds that these rules don’t just apply to the shidduch process prior to the engagement but even during the engagement period there is an obligation to reveal what should have been disclosed. (That is, with regards to a hidden or serious illness et al.) Here again it’s based on the premise that the other party will accept what you say. If they won’t accept what you say, then there is no point revealing the information.
(Rabbi Shraga Feivel Zimmerman)
Rabbi Zimmerman continues the article saying that a Shadchan must say advice that is good for the individual not both parties (etzah hogenes bo), and must not transgress the lifnei ivor factor, he cites examples of the Brisker Rov and Rav Zvi Peasch Frank to illustrate this.