Obviously one would need to analogize to the situation of a friendship, but, for example, see Rule 1.6(b)(2):
A [confidant] may reveal information relating to the [confidential conversation] to the extent the [confidant] reasonably believes necessary:
(b) to prevent the [confiding party] from committing a crime or fraud that is reasonably certain to result in substantial injury to the financial interests or property of another and in furtherance of which the [confiding party] has used or is using the [confidant]’s services
The line they draw there is that if the client/confider is, by confiding the information, involving the attorney/confidant in the misconduct or using the attorney/confidant to help perpetrate the misconduct, then the attorney/confidant is not obligated to keep the information confidential.
I feel like legal ethics has actually come up with a reasonable policy here. Check out ABA Model Rule 1.6, governing Attorney-Client confidentiality: https://www.americanbar.org/groups/professional_responsibility/publications/model_rules_of_professional_conduct/rule_1_6_confidentiality_of_information/
Obviously one would need to analogize to the situation of a friendship, but, for example, see Rule 1.6(b)(2):
A [confidant] may reveal information relating to the [confidential conversation] to the extent the [confidant] reasonably believes necessary:
(b) to prevent the [confiding party] from committing a crime or fraud that is reasonably certain to result in substantial injury to the financial interests or property of another and in furtherance of which the [confiding party] has used or is using the [confidant]’s services
The line they draw there is that if the client/confider is, by confiding the information, involving the attorney/confidant in the misconduct or using the attorney/confidant to help perpetrate the misconduct, then the attorney/confidant is not obligated to keep the information confidential.