If the violations are clear and it is worth the time and effort to inform the IRS, probably should avoid the appearance of impropriety by just informing the IRS without letting the other side know ahead of time. Maybe the better option is to wait until the suit is litigated, and then inform the IRS
How the information was obtained would need to be reviewed carefully to see if everything complies with the IRS whistleblower status. Telling the other party could be considered a threat. I would not threaten, but just take the info to the IRS if it is worth it.
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