Re notpashut:
Guard Your Tongue chapter 3 deals with giving the benefit of the doubt. The next chapter deals with someone who transgresses mitzvos between man and Hashem. Yes, it specifically states on p. 59, 9. Shaming a Willful Transgresser that one is permitted to tell others about his misbehavior, shame for his wongdoing, and that one should NOT give such a person the benefit of the doubt.
HOWEVER, while according to the letter of the law, one is permitted to speak lashon hara about such a person if one doesn’t censure him first, it is still proper to rebuke him before you publicize his wrongdoings.
IOW, one should NOT speak lashon hara unless one has first given such a person mussar. And you know how crucial it is to be sure one does that properly, with love in one’s heart and the person’s best interests at heart.
Turn the page, p. 60, where there is a brief outline of the conditions that must be met to be able to speak against such a person. You must witness it yourself unless it is established; you must be absolutely sure it was a transgression; you are not allowed to exaggerate; you must have beneficial intentions; you must not derive pleasure from this.
NOW elsewhere and I can’t spend more time on this, the Chafetz Chaim talks about protecting our CHILDREN from bad influences and IIRC it may even be permissible to exaggerate slightly there. But I think there is a difference between a kid, with whom one’s child spends a lot of time, and an adult family friend. If it is possible to talk about the family friend in tones such as nebach, they are going through nisyonos, B”H we have a solid footing etc. then for sure that would be the correct approach, I would think. And if one had any hopes of maybe being mekarev the friend, then for sure one would speak and deal with them lovingly.
Also, question here: there is no obligation to give a person the benefit of the doubt, in fact one should not. But does that only apply to mitzvos bein adom laMakom, or all mitzvos?