There is no legal obligation to report income tax fraud. So, you cannot get in trouble for not reporting him.
I would be concerned if you signed the return with him. If so, then you signed a return indicating under penalty of perjury that all the items on your return are true and accurate to the best of your knowledge. You could be on the hook as well.
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I'm not sure you will encounter trouble, though you might lose your right to assert you shouldn't be liable for any tax associated with this matter by virtue of being an innocent spouse. There are a number of variables here. You'll want to consult with a tax attorney whose practice focuses on tax controversies.
There is no duty for you to report the alleged tax fraud of your ex-husband (or the fraud committed by any person, for that matter). In determining whether it would be wise to report your husband to the IRS, I would consider such factors as whether you and he filed joint tax returns during the years that he engaged in tax fraud. If you filed joint returns in those years, then you would be jointly and severally liable for the tax deficiencies, unless you could establish to the IRS's satisfaction that you were an innocent spouse as to those liabilities. You also should be mindful that your ex-husband may deduce that you were the source of the referral to the IRS and you could face reprisal from him.
I understand your desire to see your ex-husband get his comeuppance. However, you want to make sure that it is not done at your expense. Before deciding what course to pursue, it is imperative that you meet with experienced tax counsel and explain all relevant facts to him, to enable you to make an informed decision.
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