There are 2 types of major problems that if someone knows about a shidduch he should tell the other side before they start going out, even if they are not confronted about it. 1. If the guy has a physical ailment, disease etc. a serious medical issue. Not just that he is physically a weak person because many yeshiva bachurim are physically weaker, for example. Chazal say Torah learning weakens a person physically. But a serious illness which the other side does not know about which can’t be seen from the outside should be told to the girl or boy’s side if you know they will listen and not go through with the shidduch. Or at least ask a question of advice to someone before taking the next step. Of course one has to meet all the prerequisites:
A. First see if the ailment is really a problem, not that he or she is just frail.
B. Don’t exaggerate the problem more than it is.
C. Give over information to the other party for the sake of helping and only assuming they will heed your advice not because you hate the guy or girl you are talking about. The terms of trying to find another solution before speaking up don’t really apply because they have to be told not to go through with the shidduch if it’s a major concern.
Another major concern that can be shared is whether there are signs of apikorsus. That, even on a very minuscule level there is something wrong with his fundamental belief in Torah Judaism. In that case if you know it’s true then you have an obligation to tell the other side and all rules are off because he is not considered part of “your nation” anymore. If you only hear about this secondhand, then you have to say you heard about this but not sure myself if it’s true so you should be concerned and look into the matter. Just as he himself is only allowed to be concerned and not actually believe it if it’s secondhand knowledge. However, one must be very careful because there is a difference between a lack of belief in the fundamental beliefs of Tofah Judaism and not being fully Torah observant or lacks in some areas of halacha where he is still considered part if “your nation” and speaking out about a lack of Torah observance will most likely result in lashon hara and certainly would need all the prerequisites if somehow it is permitted to be told. The same holds true about the girl’s side, if she or her household seem to be or act grossly immodest then there is no issue of telling the boy about it as long as you know he will not go out or at least ask a question if he should. If he’s going to date her anyways then it lashon hara to tell the boy.
In terms of Torah knowledge, one cannot go over to the girl’s side and tell them the boy isn’t as knowledgeable as you think or is a slow learner etc. However, if they inquire and they have every right to inquire, and back in the day people would have someone test the boy before going out with their daughter. That is fine and the tester has to be honest with the girl’s parents when he reports back. Of course, this would all be within reason. Meaning the standards that a Rabbi or Rosh Yeshiva is expecting for his daughter will probably be higher than a regular layman, so if the layman asks for the a smart guy in the yeshiva, even if he isn’t the top guy then you can still say he is very knowledgeable, but if a Rosh yeshiva asks for the top guy of some other yeshiva and the tester doesn’t say he is not the top guy then there is an issue of dishonesty because there are higher expectations. Also, if the girl’s father is going to be supporting, he wants to support someone of the caliber he is asking for, for his daughter.
One other major issue what if one knows the girl’s father cannot or will not give the support, he claims he will give. Let say for example you know her family is in fact poor, or the father is a miser, or the father even told you he is not going to be giving as much as he promised. Then before telling this to the guy there are 3 conditions that must be met:
1. Make sure the girl’s father really is trying to trick the boy because there are many times where a person who is not so rich and promises a lot of support is prepared to work very hard to fulfill his promise and this father-in-law can be better than a rich father-in-law.
2. You have to know that the boy would really be uninterested in the shidduch if he finds out he is not getting the support he was expecting. Because if he doesn’t care so much and would marry her anyways, he’ll just get any help he can get from his in-laws then you can’t say anything.
3. You have to make sure the boy isn’t hiding something and tricking the other side also. Of course, you can’t exaggerate the trickery and can’t say it out of hatred just to help. At the end of the day, one has to be very careful and not rush into revealing information, rather thoroughly think through all the facts and information, meat all the conditions and then decide to speak up or not.