The story suggests someone used your work without the business courtesy of engaging your services or explaining why they weren’t.
If you’re willing to speak with the certified public accountant, what else could you say beyond suggesting the CPA maintain her malpractice insurance, might want her own advice about whether to disassociate from further tax returns and other professional-conduct points, and might consider suggesting that the employer lawyer-up.
Or if you are willing to offer services to support corrections, suggest that the employer’s lawyer engage you, to maximize evidence-law privileges for confidential communications.
And you’d require a much-more-than-you-estimate advance retainer.
Among other cautions, don’t you want to test whether the employer is serious about corrections?