From In re Urethane Antitrust Litig., 2009 U.S. Dist. LEXIS 60496 (D. Kan. July 15, 2009):

Work-product protection is waived when privileged documents are voluntarily disclosed to an adversary because "[d]isclosure to an adversary is clearly inconsistent with the rule's goal of promoting the adversarial system." "Such a waiver occurs even when disclosure is made during the course of settlement negotiations."

[Footnote 39] Khandji [v. Keystone Resorts Mgmt., Inc., 140 F.R.D. 697, 700 (D. Colo. 1992)]; see also In re Chrysler Motors Corp. Overnight Evaluation Program Litig., 860 F.2d 844, 846 (8th Cir. 1988) ("Disclosure to an adversary waives the work product protection as to items actually disclosed, even where disclosure occurs in settlement." (internal quotation marks and citations omitted)); Grumman Aerospace Corp. v. Titanium Metals Corp. of Am., 91 F.R.D. 84, 90 (E.D.N.Y. 1981) ("Disclosure to an adversary waives the work product protection as to items actually disclosed, even where disclosure occurs in settlement.").

"The mere fact that opposing parties may have a common interest in settling claims does not neutralize the fact of disclosure, because that common interest always exists between opposing parties in any attempt at settlement."

[Footnote 40] Khandji, 140 F.R.D. at 700; see also In re Chrysler Motors Corp., 860 F.2d at 846--47 ("The fact that [the negotiating parties] may have shared a common interest in settling claims . . . does not neutralize the act of disclosure because that common interest always exists between opposing parties in any attempt at settlement.").

Moreover, when work-product protection is waived as to one adversary, it is waived as to all adversaries.

[Footnote 41] McCoo, 2000 WL 307315, at *2.