When someone I don’t know calls me for information I don’t feel obligated to say everything I know about someone. Why should I? How do I know who they are and what they are hiding? Maybe its a perfect shidduch. It is not my job to spread L”H about anyone.
On the other hand if someone I do know calls me about a shidduch and I know something about the prospect that won’t sit well with them. I tell them it is not for you. If they push me and ask why, I simply say “you called me for information because you trust me. I know you and I know that this is not what you are looking for. It is not important why I say this because it is not important for you to know what I know. It will be a perfect shidduch for someone else and people are entitled to their privacy. For you, it is not a good shidduch.
I know a lot of young people who have B”H recovered from their teenage years. Today they are wonderful bnei Torah, you would never recognize them from 5 or 6 years ago. Does that mean they shouldn’t get married? They need to be judged on who they are today, not who they were back then. But some people understand that and some people just don’t. Is it necessary for me to tell someone about a particular person’s past and spread the information or is it enough if I just say “the shidduch is not for you” knowing that they would never forgive the prospect’s past?
If someone asks me I know this person had a very troubled childhood what are they like today? I can be truthful and honest with them and tell them what I know about that person.