Reprimanding a parent for transgressing Jewish law:[1]

One who witnesses his parent transgressing words of the Torah should not explicitly tell him, “You have transgressed the words of the Torah.” Rather, one is to say, “Father, is it not written in the Torah such and such?” making it as if one is asking the parent a question, rather than warning him [and accusing him of transgression]. In this way one can have the parents understand on their own [that they are doing a transgression], without needing to embarrass them in the process. [Alternatively, one can respectfully ask his parent as to the source of the allowance for him to do such and such.[2] However, if this respectful tone does not work, then one may use a stronger tone to separate his parents from doing a prohibition.[3] However, if one knows that his parent will not listen to him, and it is not a matter that is explicit in Scripture or that is a known prohibition, then it is forbidden for him to reprimand his parents actions, as it is better that the parent be a negligent sinner then an advertent sinner.[4] Likewise, if the parent is transgressing only a matter of custom or stringency, or matter that is disputed amongst the Poskim, that he may not reprimand them.[5] However, if one assesses that the parent is simply making an innocent error in his judgment, then it is obvious that one may respectfully mention it to one’s parent. Certainly, if one knows that one’s parent will rejoice in being told the correct law, such as if the mistake will lead him to be shamed, then one may inform him even regarding mundane matters that do not relate to Torah and Mitzvos. Thus, if one’s parent wore his shirt backwards, it is a mitzvah to inform him in order so he not be shamed in public. All in all, regarding such matters one is to act with common sense and judge the matter appropriately.[6]]

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[1] Michaber 240:11; Rambam Mamarim 6:11; Kiddushin 32a [that if one’s father was seen to be transgressing words of Torah one should not tell his father that he has transgressed Torah but rather is the tell him that there is a verse in the Torah which states such and such. He should not even say the Torah says such and such as doing so would embarrass him.]; Sanhedrin 81a; Pesakim Uteshuvos 240:38; See Encyclopedia Talmudit Erech Morah Av Vaeim Vol. 42 pp. 589-591

[2] Betzel Hachochmah 3:21-4

[3] Rishon Letziyon 240:11; Chidrei Deiah 240:8; Chofetz Chaim Kelalei 10 Halacha 4; Betzel Hachochmah 3:21-4

[4] See Rama 608:2; Admur 608: M”B 608:4; M”B 303:82

[5] See Sefer Hamakneh Kiddushin 32a; Pesakim Uteshuvos 240:38 footnote 341

[6] Pesakim Uteshuvos ibid

[7] Likkutei Sichos 22:214, printed in Shulchan Menachem 4:176