The Rebbe's guidance on truthfulness, honesty and constructive criticism:
Report the truth. Never apologize for reporting the true situation, even when it is not ideal; concealing the truth is improper and causes needless distress (IK 5141). Accurate reporting is needed so that proper guidance can be given, and asking for it is meant to strengthen, not diminish, anyone (IK 2052).
Constructive criticism. Shortcomings should be reported, because honest communication is the path to improvement — but constructively and without exaggeration, and rebuke, when needed, should be persistent and sincere (IK 6663). Criticism of a teacher or speaker belongs in a private conversation, not a public challenge that embarrasses him (IK 7172).
Integrity in judgment and in public. A judge must investigate thoroughly and rule with absolute integrity (IK 3289). Misattributed public statements should be corrected to prevent misunderstanding (IK 10165), and a publication should reflect authentic Judaism without compromise (IK 10873).
See also: Between people – Guarding one's speech and judging others favorably; Leadership – Leading and organizing communal work.