It depends. I am assuming he was a US resident for tax purposes prior to departure.  How long was he a resident?

If he was considered a US Resident for tax purposes prior to his departure, he can make an election to terminate the residency on the first day that he was no longer a lawful permanent resident (resulting in dual status alien treatment) or by default be treated as a resident for tax purposes for the entire year.  

NOTE:  Depending on length of residency, net worth and average net income he may be subject to expatriation tax.  

US Tax Guide for Aliens