The fraud allegations against a former Willkie Farr & Gallagher partner that emerged last year—that she and her husband had bilked the firm out of millions—were shocking enough. But in August the damage got worse, when evidence in the fraud case scuttled a major antitrust settlement between American Express Co. and retailers.

Emails unearthed in the fraud case revealed that a plaintiffs lawyer in the AmEx case, Gary Friedman, shared confidential documents with Keila Ravelo, counsel for Mastercard Incorporated in a similar case when she was at Willkie. Ravelo resigned from the firm last November.