Manigault is credited with saving Duncan's life by administering CPR after he went into cardiac arrest at home July 13.
He was hospitalized until Monday when he died of what doctors suspect was a second heart attack. The star of "The Green Mile" and "Armageddon" was 54.
"She was literally there every second," a source close to the couple told People.com of Manigault. "She is a strong woman who is full of faith, but right now she is heartbroken."
Manigault worked in Vice President Al Gore's office during the Clinton administration as deputy associate director of presidential personnel but is best known as an outspoken contestant on the business-themed competition TV series "The Apprentice." An ordained minister, she first confirmed she was dating Duncan in 2010.
Duncan's death is the latest personal tragedy she has suffered. Her brother was shot to death last year and her father murdered when she was 7, Wikipedia said.