Best source is not a third party's article that doesn't describe the requirements for voluntary disclosure, but IRS itself:

https://www.irs.gov/compliance/criminal-investigation/irs-criminal-investigation-voluntary-disclosur...

One key point is that the taxpayer should contact IRS, before IRS contacts the taxpayer.  So I would want to know why this person suddenly got religion.  I don't think the facts you describe rise to the level of a criminal prosecution, so the question becomes whether to drag out the process or come clean quickly. Although IRS is unlikely to prosecute the taxpayer, they might be interested in going after the preparer, and they would need witnesses to testify before a grand jury and at trial.