Before speaking about a person with others, one must first discuss the issue with that person himself. Trying to persuade the sinner to change his ways is a course which must be pursued before bringing anyone else into the picture. (This would not apply if speaking to the sinner might make it difficult, if not impossible, to achieve the desired result through any other means.)
(Taken from the Sefer Shmiras Halashon, Chofetz Chaim A Lesson A Day 15 Chesvhvan – Day 44 pg. 138)