Shemiras Halashon Halachos
Illustrations paragraph 6 Now we will go back to explain what we permitted at the beginning of paragraph 4. If the bridegroom has great deficiencies, there is a distinction. If the deficiency is an illness which the other side does not know of because it is internal, it is obviously not rechilus to reveal this (if all the details are fulfilled).
There is another case where one must reveal the facts – if one heard that the bridegroom is a heretic. Regarding this one can say the Torah writes, Do not go rachil in your nation, right before, Do not stand by your fellow's blood. But if the deficiency is that the bridegroom doesn't know much Torah, one does not have to reveal this as the father-in-law caused the loss to himself. He should have got a learned person to test his knowledge of Torah. If not, he accepted the bridegroom as he is.
Illustrations paragraph 7 The same applies to telling the bridegroom about the father-in-law. If the bride has a indiscernable internal disease and the bridegroom would not agree to the marriage if he knew of it, one may tell him if the conditions are fulfilled. The same applies if the father-in-law's home is immodest, which is great reason for the purpose of marriage to be destroyed. However, one should consider whether the bridegroom will listen to him, for it often happens that he will not and telling him only leads to rechilus.