Disclosing privileged communications to third parties generally waives that fragile protection, even if the third parties are friendly. In contrast, disclosing work product to third parties waives that more robust protection only if it increases the chance of the disclosed documents "falling into enemy hands."
In In re Grand Jury Matter #3, 847 F.3d 157 (3d Cir. 2017) (per curiam), the Third Circuit affirmed these key principles in a common context – disclosure to an accountant. The court held that although the subject of a criminal investigation "waived the attorney-client privilege by forwarding [a privileged and work product-protected] email to his accountant, the document still retained its work-product status." Id. at 165.
These dramatically different waiver implications apply when clients disclose documents deserving both protections to other friendly third parties -- such as public relations consultants, banks, business advisors, family members, etc.