Sounds like he's a green card holder and he's got a re-entry permit to work temporarily away from the US.  As a green card holder, he is subject to US tax on a worldwide basis, just like any American.  If he doesn't comply with his US filing and reporting requirements (including various information returns) and pay his taxes, that could put his green card in jeopardy.  Foreign earned income exclusion and foreign tax credit should help alleviate double taxation.