Tena Marie Broadus, 29, died over vending-machine change, witness Devin Deshawn Gregory admitted Wednesday in Circuit Court.
Gregory pleaded guilty to second-degree murder before Judge Christopher Schmidt.
Gregory will be sentenced at a later date. He is first expected to testify, if needed, against the suspect charged with first-degree murder in the case.
Gregory told the judge what happened the day Broadus, who was from Gulfport, died in September 2015.
Gregory and others were hanging out at the Gulfport home of Joshua Anthony Peterman, a "big-time" member of the Simon City Royals, according to an attorney in the case.
Gregory said they had been snorting, smoking or shooting up methamphetamine for four days. Several days earlier, he had been riding around with Peterman, 31, and Broadus.
Peterman was stealing coins from vending machines. The Gulfport police attempted to pull over their vehicle when Peterman crashed and ran on foot from the scene.
Police questioned Gregory and Broadus, said Gregory's attorney, Steve Simpson. But the two did not give officers Peterman's name.
Peterman was still worried Broadus was going to give him up on the misdemeanor charge.
At the house, Broadus and Peterman's girlfriend, 36-year-old Kari Michelle Parker, started to argue, Gregory told the judge.
By the time the fight wound its way to the yard, Gregory said, both Parker and Peterman were hitting Broadus. Peterman, who had a machete with a serrated blade, told Gregory to get some duct tape, so he did.
Broadus was taken into a shed. "I hear a whole bunch of banging going on in the shed," Gregory said. When he went inside, he said, he saw Peterman and Parker hitting Broadus on the head.
Broadus was duct taped by the arms to a chair.
"I said, 'Bro, you've got to let this girl go,' " Gregory told the judge.
The couple eventually left Broadus in the shed, Gregory said, and everyone went inside to get high.
The last time he saw Broadus, Gregory said, she was tied up with rope hanging from a beam. He said he left to fetch a methamphetamine pipe from his house.
When he got back, he said, Parker told him that she had slit Broadus' throat.
"I realized they really killed a girl," said Gregory, who has for years suffered from mental illness but is on medication and was deemed competent to stand trial. "I thought they were playing. (Peterman) had done that before — beat girls up and tied them up — but he usually let them go."
The next morning, Peterman picked up Gregory at his house. Gregory, by this time, feared for his life. He thought he would be next to die, he said.
"Please don't kill me," Gregory said he kept pleading. "Please don't kill me." Peterman assured Gregory they were like brothers and he would not die.
Instead, they drove to the woods. Gregory saw a barrel with smoke coming out of it. Inside the barrel, Gregory said he saw ashes and a skull. Peterman, he said, took the skull and threw it in the water.
Investigators later found Broadus' remains in the Biloxi River.
Gregory said he later got into a fight with his mother at home, then tried to hang himself from a noose in his bedroom closet.
Attorney Simpson told the judge that Peterman has been sending threats against Gregory in the jail. The District Attorney's Office planned to make sure Peterman would not be in a position to threaten Gregory further.
Peterman is scheduled for trial May 7 on a first-degree murder charge. Parker pleaded guilty in September to second-degree murder
Outside court, Simpson said Gregory did not see Broadus killed. He said Peterman claimed to someone else that he had done it.
Simpson said, "My client doesn't know who did it."