Sources said Lisa Blue Baron - wife of late Democratic fundraiser Fred Baron - was scheduled to testify before a federal grand jury that's looking into Edwards' affairs.
Baron - who died in 2009 - was a Texas attorney who managed the Edwards' campaign finances.
The grand jury has been investigating the former North Carolina senator and 2008 presidential candidate for more than a year.
He has denied any wrongdoing.
According to sources, prosecutors are focused on the possible misuse of campaign contributions. They want to know about the arrangement to conceal the affair and the baby John Edwards had with his mistress, Rielle Hunter.
Sources say prosecutors are gathering information about the people who donated money to help Edwards' former aide Andrew Young, his wife, and Hunter live a life on the run.
Both Hunter and Young have testified before the grand jury.
Hunter worked for Edwards' political action committee in 2006, shooting behind-the-scenes video as the Democrat prepared to launch his White House campaign. The committee paid her video production firm $100,000 that year and then another $14,000 later on in what a senior campaign official described as a payment for leftover footage.
Young has said he was tasked with taking care of Hunter both during the campaign and after. He described receiving hundreds of thousands of dollars from Edwards supporters.
Edwards has largely been in seclusion since admitting two years ago that he had an affair with Hunter. He publicly admitted for the first time earlier this year he fathered her child. John and his Elizabeth separated after the announcement.
She lost a long battle with breast cancer last month. John is not mentioned in her will.