One of my friends who is US Citizen (born in US/white) married a Korean girl, but one time when they travelled they came back to US separately and the US immigration officer thought that the girl might be a suspected fraudulent case so they denied her entry. So my friend actually had a buy a ticket for her to go back to her country. She was pregnant at the time and now she also had a baby but immigration is still not allowing her to come back.

Anyone have any idea on what can be done?

Write emails and call your US Senators (both senators from your state) and your district's U.S. Congressman.

This is one of the precise reasons we elect them to represent us at the federal government level.

Must consult an Immigration Lawyer.

Why are you posting for a guy who was born american citizen, not migrated immigrant turned citizen like we will be some day (hopefully). He should have more knowledge in this matter since marrying outside of country requires a lot of paper work and especially korean girls are more scrutinized than other nationality since there has been a lot more cases of them marrying US citizen and divorcing them here (even if guy was not aware or was not part of this scam and wanted to stay with girl). Since one of my friends is deployed in South Korea, he told me all those horror stories regarding american soldiers and korean girls using them to get into country.

Getting an immigrant visa for spouse is a privilege not a right of U.S Citizen. Immigration can deny the visa or entry for any reason they want. He can definitely try to explain them on detail and try to convince them that the marriage is not a fraudulent one.