While the friend was seeking help, the attacker pointed his gun at the woman he was arguing with and shot her dead.
Once police responded, Lopez ran into the hospital before shooting at law enforcement as well as innocent bystanders, including "a poor woman who just came off the elevator" - presumably Less, the pharmaceutical assistant.
Lopez and O'Neal had set a wedding date of October 27, according to the former couple's gift registry at Bed, Bath & Beyond. She was a recent Purdue University graduate.
A gunman opened fire at a hospital in the U.S. city of Chicago on Monday, killing four people, including a Chicago police officer, a doctor and a pharmacy assistant at Mercy Hospital on the city's Near South Side.
Her father, Brian Less, told reporters on Tuesday: "She was forged in her own adversity, which made her the strongest person I will ever know". In the attack at least six people were injured, one was killed.
Cook County Medical Examiner's Officer later confirmed that they had both also died. O'Neal had recently called off their engagement, and Lopez confronted her about returning the engagement ring, police said.
The police officer, Samuel Jimenez, was assigned to the second district and entered the hospital when he heard gunshots.
"Chicago Police Department have secured the hospital and patients are safe", the hospital tweeted.
The emergency room doctor, identified as 38-year-old Tamara O'Neal, appears to have been chosen as a victim simply because she used to be engaged to the gunman, 32-year-old Juan Lopez.
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Authorities said the shooting began as a "domestic dispute" between the gunman and O'Neal in the hospital parking lot.
James Gray said he was inside Mercy Hospital when the shooting unfolded.
Inside the hospital there were scenes of chaos as he fired at will and managed to reload twice.
"We just don't know how much damage he was prepared to do, " Superintendent Johnson said, adding that Less "had nothing to do with nothing".
When a friend of O'Neal's tried to intervene, "the offender lifted up his shirt and displayed a handgun", Chicago Police Superintendent Eddie Johnson said.
During a news conference Tuesday afternoon, Stephanie Loudin, the nurse manager at the hospital, said O'Neal started as a resident at Mercy after graduating from the University of IL at Chicago and became part of the medical staff in 2016.
Chicago police say the suspected gunman was later killed.
Officers locked down the hospital's first floor and closed off stairwells, eventually locating the shooter, who had sustained a fatal head wound. He then fired multiple shots at officers who arrived before they could exit their squad vehicle.