If he files a 2019 tax return as Married Filing Jointly without her consent, then he is filing a fraudulent tax return and committing perjury.
When signing a US federal tax return, the following is stated on the form -
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
To report tax fraud - https://www.irs.gov/individuals/how-do-you-report-suspected-tax-fraud-activity
