Hunter had started out eager just to be around Edwards, but over time became more comfortable in her role as Edwards' lover -- even wife -- having sex in the Edwards' marital bed, according to Young. Eventually, she became possessive and demanding, Young claims.
When Edwards rushed home in tears from campaigning in Iowa at the news that his wife's cancer had returned, he used Young's phone to call Hunter to cancel a date to celebrate her birthday in Des Moines that night.
"All I could hear was Rielle cussing," Young said. "She [Hunter] didn't care about Elizabeth's prognosis. All she cared about was that the senator was not going to be there to celebrate the birthday."
Each time Edwards professed his love for his wife on the campaign trail, Young said, "Rielle would go crazy...and it was my job and Cheri's job to calm her down."
The stakes got even higher in May 2007 when Young said he got a frantic call from Hunter.
"She said, 'I need to talk to him right now,' and started cursing and she threatened to go public if I didn't put them together. I said, 'well, either somebody's died, or somebody's pregnant.' And she said, 'Well, nobody's died,'" Young recalled.
Young said Edwards was shocked by the pregnancy and believed there was only a one-in-three chance that the baby was his.
"He was cussing her out, calling her crazy ... and saying that ... she had sworn to him that she was physically unable to get pregnant. And that he just felt like he had been set up," Young said.
Young also maintains that Edwards had also asked him to arrange a fake a paternity test. "Get a doctor to fake the DNA results," Young said Edwards told him. "And he asked me ... to steal a diaper from the baby so he could secretly do a DNA test to find out if this [was] indeed his child."
Young said he ignored the requests.
According to Young, Edwards looked to abortion as a way out but soon planned a cover-up to hide Hunter from the hawk eyes of the media and his cancer-stricken wife, Elizabeth.
"There was a whole series of calls of how do we deal with this," Young said. "This was no longer an affair. This was going to be, you know, at least, at the minimum, a year-long commitment. And it was going to take much larger resources than we had."
ABC News' Michael S. James contributed to this report.
Watch "20/20" Friday at 10 p.m. ET and visit the "20/20" Web site Friday to follow the money trail of Edwards' benefactors, who provided cash, private jets and secluded mansions that were used to hide Edwards' pregnant mistress.