Fair question - first off, thank you for posting. In my experience, tax accountants often are too scared to discuss issues, and they are the most likely to get into trouble.
But, agree with the comments above - it seems highly unethical for a tax preparer to even think about being a "whistleblower". As a professional, your obligation is to prepare returns correctly, and to inform taxpayers when you identify an issue with a prior return (and it's good practice document that communication in your files).
Regarding the question - taxpayers are permitted a fair amount of flexibility in calculating taxable income, to even assume that a prior year return was flat wrong (short of a situation like getting W-2 amounts wrong) is not fair to the taxpayer (and to the tax preparer's intellectual curiosity) just because it's not clear regarding the accounting used for the prior return. I don't see where the income "misstatement" issue arises without knowing what the item is. Balance sheets, are often messed up, but that doesn't mean income is incorrect. In addition, an "accrued" item or a "payable" DOES NOT necessarily mean that a cash-basis taxpayer couldn't take it into account; quite the opposite, the Tax Code is loaded with rules where a taxpayer's overall method of accounting does not determine the treatment for a specific item (cash vs. accrual, special methods, etc.). In addition, taxpayer's are allowed to elect treatments for many different items which change the timing of income/expense recognition. As a general rule of thumb, I would start with trying to figure out why the position is permissible, rather than why it's impermissible.
Taxpayer's often have no clue regarding accounting - if it takes me more than a sentence or two to clearly explain the issue, that means I don't understand the issue well enough; that's a failure on my part, not the client's.
Just some thoughts, thanks again for posting.