-Only one of the parents is US citizen is enough.
It the kid was an orphan before he/she was adopted by your friend, this kid can get the US citizenship after he/she is admitted into U.S. with IR-3 visa. But if the kid's biological parents are alive, the situation is very different. Please see the left icon on this webpage for some basic idea: "關於'領養'親屬子女和移民問題"