"I was hoping to find a rule, a law, a tax code"

The reason you don't have a Hard Line is that you seem to be trying to apply the concept of Statute of Limitations to tax filings. That doesn't apply to filing taxes. There is no "safe at home if I didn't file X and they didn't catch me by Y."

Read these articles:

https://www.thebalance.com/back-taxes-filing-late-tax-returns-3193548

https://www.freshbooks.com/hub/taxes/how-far-back-can-irs-go-unfiled-taxes

https://www.thebalance.com/when-you-haven-t-filed-tax-returns-in-a-few-years-3193355

Basically, your client is playing russian roulette = how many spins are on that chamber before a round ends up in the barrel and the IRS has reason to pull the trigger?