Pack to the topic at hand
"So, if you know someone is defrauding the IRS and send them a letter, you can be sued for that?"
Why bother with this petty revenge thing when there's money involved? And do you really think the employer will appreciate being told to do the IRS's job?
Contact the IRS criminal investigation division, ask for the forms to be registered as a confidential informant and then provide them with the evidence they need to investigate this crime. If the eveidence is good enough to warrant an investigation and results the IRS getting $$ from the guy, you may be eligible to get a percentage of the $$