Thanks to everyone for responses. I wanted to update on how things worked out. My friend was able to re-enter the country with no problem at all - no additional questioning, no problem not having the GC with him. He was asked why he doesn't have it with, and he said that he left it at home cause he didn't want to lose it (not sure why he said that). To which the officer said that it wasn't a problem. BTW, I contacted the CBP (Customs & Border Protection Agency) before his return with this question, and they told me that he might be sent to a "deferred immigration check", which I believe means that upon return home he'd have to come to the Immigration office to show his GC. This wasn't necessary. Just thought I'd share this info for other people who might be in a similar situation.