Hilchos Rechilus, chapter 7, section 1
Rechilus is forbidden for man and woman, relative or non-relative. Even if one heard someone speaking against one’s parents and revealed to this to them because of one’s pain, this is also rechilus. It makes no difference if the person spoken of is a man or woman, adult or child, as we explained earlier in part 1 chapter 8 paragraphs 1,2 and 3.
Some people err when they see two boys hitting each other by telling the father of one of them how someone hit his son. This leads to countless prohibitions. The father hits the boy who hit his son and that leads to controversy between the two families, and it is very common in places of Torah study. If one doesn’t know which boy was in the right it is certainly forbidden to speak to a father of one of them. Even if one knows that one is in the right, one may not tell his father unless the conditions explained later in chapter 9 are fulfilled.