My brother changed his first name to his nickname when he turned 21 and his last name when he got married (don't ask). His passport doesn't match his green card because the Dutch government doesn't recognize name changes, but the US gov't does. He flies internationally without much problem, although he is asked occasionally for some proof of the name change. I suspect if your friend has documentation that shows both names he will be ok. He may need to answer more questions than otherwise. of course, if he can get the name on the ticket chnaged before flying he'd be better off.