Illustrations paragraph 8
If he sees that the father-in-law is cheating the bridegroom regarding the dowry or support, one must be very careful before telling the bridegroom as a number of conditions must be fulfilled:
a) One must make sure he is really deceiving him and personally know that due to his evil nature or poverty he will not pay, or have heard him say he will not keep his promise.
b) One must clearly know that if the bridegroom knew of this he would not go ahead with the marriage. For the bridegroom often only wants to get what he can from the father-in-law but does not make the marriage dependent on this. In such a case one cannot tell him (unless it causes the bridegroom to make the father-in-law appoint guarantors for his obligations).
c) If the bridegroom is also deceiving the father-in-law, the two cancel each other out and it is obvious one cannot tell him.
Besides these details, one needs to fulfill the regular conditions as well. Therefore, one must not hurry to inform him until one has though well about the matter.