Fascinating question. On a simpler level, sometimes not telling someone about a Halachic problem if you know they won't follow the Halacha, is definitely the Halachically correct thing to do. For example, if you know the Eruv is down and you see someone carrying, if you know they will do the right thing (which might be leave the item where they are or might be to turn around and walk back to their origin location and leave the item there) then you tell them, but if you believe that they will ignore you or deny it or for whatever reason continue carrying, you generally don't tell them because better they should be unknowingly violating (thinking the Eruv is up) than deliberately violating. But to your specific question, there are a number of stories where things like this have happened and the Baal Teshuvah tells her mother, thinking it will be a big problem and gets a response of "Yes, we're Jewish, I've hid it all these years because the family was always scared."

Oh, good point -- I had forgotten that for specific accidental transgressions, there's guidance (I didn't know it was the agreed halacha, but I've heard the advice) to not say anything if you know the person will ignore you. An ongoing situation might be different, but I don't have good instincts here, nor do I know where to search, hence the question.