Before assuming that a person’s actions merit tochachah, reproof, one must be absolutely certain that he has, in fact, sinned.
From the command: Judge your fellow favorable, we learn that if a person appears to be committing a transgression which is totally out of character for him, we are to seek to understand what transpired in a positive light. If there is any way to interpret his actions as not involving transgression, we are required to do so.
If it is absolutely clear that the person did sin in a manner which is out of character for him, then we must assume that he immediately regretted his actions, and has already repented. To discuss the incident with anyone constitutes speaking loshon hora.
Sefer Chofetz Chaim, 22 Cheshvan, page 152