Protesting someone who defames one’s parent:
It is a mitzvah on one who hears a person defaming his parent to protest against him, and to defend his parents from the libel. A child does not have the right to forgive his parents honor in these matters. Nonetheless, if one knows that by protesting the defamation it’ll cause it to be spread even more, then it is permitted for him to remain quiet.
Not to befriend those who speak against one’s parents:
One is to distance himself from people who speak against his parents or belittle them, and he should therefore avoid speaking with them.
 Pesakim Uteshuvos 240:24
 Sefer Hayirah of Rabbeinu Yona 197; Sefer Chassidim 72; See Orchos Chaim of Rosh 82
 Yam Shel Shlomo Bava Kama 8:50; Shut 101
 Sefer Hayirah ibid; See Minchas Elazar 4:13
 Birkeiy Yosef 334:16 in name of Mishpat Tzedek; Pesakim Uteshuvos 240:24 footnote 204