In the case of a habitual sinner who ignores a particular commandment altogether, one is to assume that even a questionable act is, in fact, a transgression of that commandment. “Judge your fellow favorably” demands fairness, not naivete.
Having concluded that the person did transgress, one must then seek to understand why he frequently commits this sin. It may be due to ignorance – he may be totally unaware that the act is forbidden, or he might not realize the severity of the prohibition. Should this be the case, one must find some way of enlightening the person without hurting his feelings. Discussing his behavior with others constitutes speaking loshon hora. (This situation is common among those who were raised in communities where there is widespread violation of certain laws. In such instances, people mistakenly view those who observe these laws as machmirim, individuals who are stringent beyond the letter of the law.)
Sefer Chofetz Chaim, 24 Cheshvan, page 156