if you want to report it at your own risk there is form 3949-A
https://www.irs.gov/pub/irs-pdf/f3949a.pdf
if you want to claim a reward use form 211
https://www.irs.gov/pub/irs-pdf/f211.pdf
however, I will point out it seems that you were the one that created the situation by preparing the returns so the reward could be denied and you might be visited by IRS special agents who might ask you under penalties of perjury how you came about this info
