Halakhah sur Édouyot 5:11

And how much must one strengthen himself not to give these people an opening in the beginning; that is, not to argue with them, but to strengthen himself greatly not to associate himself with them at all, as it is written (Mishlei 1:11-16): "If they say, 'Come with us… keep your feet from their path. For their feet run to evil, etc.'" And Chazal have said in Eduyoth 5:6: "Better that a man be called a fool all of his days and not be wicked one moment before the L–rd." "called a fool" is stated because of the matter discussed there; but in truth, it befits a man to commit folly, even actively, so that the epithet "wicked" not apply to him, even for one moment, above. As we find in Scripture (viz. I Samuel 21:16) that David made himself [act] like a fool in the house of Avimelech so that he not fall into his hands. How much more so [should one follow this stratagem if necessary] for the sake of the King of kings, the Holy One Blessed be He! And (Chullin 5b): "(Psalms 36:7): 'Man and beast do You save, O L–rd' – these are men who are wise in knowledge, but who make themselves [act foolish] like a beast [for the sake of the L–rd]." And see Berachoth 19b: "If one finds sha'atnez in his garment, etc."

And from this we can understand that lashon hara is certainly forbidden in an instance where only one's personal honor is at stake. As when one is sitting in a company of men and has no way of avoiding them, and they are speaking of things which are forbidden according to the din. If he sits in silence and in no way abets them in their talk, he will be regarded as "crazy." Of this and all such things, Chazal have said (Eduyoth 5:6): "Better that a man be called 'a fool' all of his days rather than be wicked one moment before the L-rd." He must harness all of his powers at that moment to withstand the trial, and [if he does so], he may be completely confident that his reward for this from the Blessed L–rd will be without end. As Chazal have stated (Avoth 5:23): "According to the strain is the reward." And, in Avoth d'R. Nathan: "One time with strain for a hundred times without strain." (That is, the reward for the performance of a mitzvah or the abstention from an issur, which entails strain, is a hundredfold more than for that of the same kind, which entails no strain.) And to such a time [of trial as the above] there certainly applies the statement of Chazal in the Midrash: "For every moment that one 'muzzles' his mouth, he attains to such secreted light that no angel or [Divine] creature can conceive of." (As to how one should conduct himself in respect to reproof and listening if one is "caught" in such an evil company as this one, see below Principle VI, sections 4-6, and above, in the introduction to the negative commandments, section 16.)

(16) And sometimes he transgresses also the issur of flattery, which is to many Geonim (e.g., HaRe'em, Baal Hatosfoth, and Hagaon R. Shlomoh ben G'virol) an absolute negative commandment, (viz. Bamidbar 35:33): "And you shall not flatter [the men of] the land." That is, if his intent in speaking lashon hara and rechiluth is to flatter the listener, whom he knows to bear hatred to the one spoken about, and thereby, to find favor in his eyes — an egregious sin — is it not enough that he does not fulfill the mitzvah of reproof (a positive commandment in the Torah), to reprove him for the hatred he bears his friend, that he also strengthens the hatred that already exists among them! And through him [the speaker], he [the listener] will persist in his wrong more and more, so that more quarrels and wrongs will result (G–d forbid)!
And know that this [the following] sin (in our many sins) is widespread. That is, when one speaks demeaningly of his friend, (often,) the listener, knowing that what is being said is unfounded, nevertheless nods his head [in agreement] and he, too, "smoothes over" the thing with his tongue, adding some words of taint. For the speaker is sometimes a man of means, or the like, from whom he receives favors, or who he fears will regard him as unwise, or the like [for remaining silent]. And, therefore, the yetzer will entice him, too, to concur in this. But know, my brother, that this, too, is essentially a transgression of the negative commandment of flattery — even if he adds but a few words — as is explained in the Be'er Mayim Chayim.
And in this regard it is written (Mishlei 23:2): "And put a knife to your throat [against speaking lashon hara] if you are a man of spirit." And one must rather expose himself to danger than bring his soul to such a sin. According to the Torah, every man under such circumstances must, in any event, strengthen himself not to abet him [the speaker] even by so much as one movement which would cause it to appear that he concurs with what he is saying. And in this connection we can understand the words of Chazal (Eiduyoth 5:6): "It is better to be called a fool all of one's days than to be wicked one moment before the Almighty." And this, even if he knows that his words of reproof will not be accepted by the speaker; for, otherwise, he certainly must reprove him for this, too (as will be explained, G–d willing, in Hilchoth Lashon Hara, Principle VI).