Before relating negative information l’toeles, for a constructive purpose, one must carefully consider whether or not his words will, in fact, bring about the desired result. Speaking negatively l’toeles is justified only if there is a reasonable chance that the desired result will be accomplished.
This point is of particular importance when one seeks to discourage a potentially harmful friendship, business association or shidduch (marriage match). Often, a final decision has already been made, and speaking negatively of the other party will serve no constructive purpose.
Sefer Chofetz Chaim, 14 Cheshvan, page 136