the IRS would come after them if they did. Most likely you misunderstood what your friends told you.

It so happens that my uncle worked for Halliburton in Peru. He had the standard expat tax package, same as I did on my expat assignments: Halliburton paid his Peruvian income taxes, while he paid US income taxes to the IRS. He filed what is called a "hypothetical" income tax with his employer, which is what he would have paid in US taxes had he remained in the US -- Halliburton then reimbursed him for the difference, so that he didn't wind up being penalized by paying more taxes by working overseas.

If your friends somehow did manage to not pay any taxes to the IRS, they'll likely be in prison soon, and their US assets will be seized