Another form of constructive speech is that which prevents or breaks up a relationship that is harmful to at least one of the parties. It is correct to suggest that someone distance himself from a person or persons who may have a negative influence upon him. Similarly, it is proper to inform a parent that someone may be having a negative influence on his child. All preconditions must, of course, be fulfilled – including verification of facts and constructive intent. If the information is not firsthand it mey nevertheless be conveyed, provided that it is clearly being presented as secondhand information.
With regard to the requirement of accuracy of the information, the Chofetz Chaim suggests that exaggeration may be permissible if that is the only way to convince the person or parent to end the relationship.
The Chofetz Chaim adds that it is imperative to explain why conveying the information does not constutute speaking loshon hora, especially when speaking to a child.
Sefer Chofetz Chaim, 21 Kislev, page 210