It is forbidden to speak derogatorily about someone even when the subject has made it known that he does not mind if such things are said of him.

As mentioned above, to speak negatively of one’s fellow is shameful in itself, regardless of the subject’s feelings. Speaking loshon hara is contrary to man’s exalted status as the only creature who was fashioned b’tzelem Elokim, in the Divine image — and this is not altered by the fact that o has been granted permission to speak derogatorily.

Sefer Chofetz Chaim, 10 Tishrei, page 68

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