Esta and green card

Question - my friend's son is married to an american and lives in the US. He has a green card, but at the moment not dual citizenship. He's coming over very shortly, will he need to do an esta for his flight back to the US? We were talking about this last night - I'd have thought probably not, but does anyone know for sure?

No, the visa waiver authorization (ESTA) only applies to citizens of visa waived countries who plan to visit the United States as tourists for a non-extensible duration of up to 90 days. Green Card holders do not need an ESTA approval and may travel to the U.S, by just providing the form I-551.

That makes sense - many thanks.