@Jemimaschmidt
If the noncustodial parent did not have a valid signed Form 8332 and claimed the child as a dependent on their tax return, then they committed tax fraud and perjury by filing a false return.
As stated in the Signature block of a federal tax return -
Under penalties of perjury, I declare that I have examined this return and accompanying schedules and statements, and to the best of my knowledge and belief, they are true, correct, and complete.
Go to this IRS website to report tax fraud - https://www.irs.gov/individuals/how-do-you-report-suspected-tax-fraud-activity
