"I thought I heard someone calling for help, so I sprinted down there," Simpson tells PEOPLE. "I pushed the door open and saw a guy bear-hugging someone to his chest."
NBA player Ryan Anderson had just arrived back at his girlfriend Gia Allemand's apartment and found her hanging from a metal staircase by the door.
"He said, 'She hung herself,' and I looked over and that's when I saw the rope," Simpson says.
A woman believed to be a neighbor dashed back into the apartment and handed Simpson a phone, saying, "Talk to 911."
By this point Allemand was lying on the floor, and the 911 operator told Simpson that they needed to do CPR. Anderson, who was crying as he leaned over the unconscious 29-year-old, started doing chest compressions.
"He just kept yelling her name, 'Why, Gia? Why?' Just sobbing," Simpson says of Anderson.
Listen to the 911 operator instruct Simpson on how to administer CPR:
Realizing that they needed more help, he screamed down the hallway, "We need a doctor!," and a nurse from a nearby apartment took over the compressions while Anderson began doing mouth to mouth.
Simpson estimates he'd been on the phone with the 911 operator for eight minutes when "an EMS guy arrived."
"He asked how long she had been like this, and the boyfriend said, 'I was gone for an hour and I came home and she was like this.' He didn't say it calmly. He said something to the effect of, 'I wasn't gone for more than an hour,' " Simpson says. "I just remember thinking, he is just so distraught."
As the EMS staff worked on the Bachelor star, Simpson left the apartment, deeply shaken by what he'd witnessed.
Memorial Service PlannedAllemand was taken to a New Orleans hospital in critical condition, and two days later was taken off life support.
Anderson, 25, who'd remained by his girlfriend's side in the hospital, issued a statement Wednesday mourning her loss.
"Gia was the most beautiful person I knew inside and out, and she always smiled and made everyone else around her smile," he said. "All I have left is to cherish those memories we made together and help perpetuate the many values, faith and love that we shared."
As her family and friends prepare to say farewell to the model and actress in a memorial service in New York, Simpson tells PEOPLE that the New Orleans Pelicans player should be commended for everything he did to try and save Allemand that night.
"It was heroic what he did, and what the nurse and that neighbor girl did," Simpson says. "He was distraught, it was his girlfriend and he was fighting though all of those demons to take care of her. I think that was the hardest part for me. It was watching him. He just kept saying, 'Gia, Gia, Gia.' "