Surveillance video shows the incident unfolding in a parking lot on the 400 block of South 69th Street on Friday afternoon.
Police said a 37-year-old man and his girlfriend were fighting when he threw her to the ground. That's when the 73-year-old man intervened with a hatchet in hand.
Video shows the boyfriend knocking the man's hat off. Soon after, the 73-year-old hit the boyfriend in the shoulder with the hatchet.
Police said the boyfriend told officers he grabbed his gun and shot the man in the foot because he was old.
"Seventy-three years old, he probably thought, 'I'm just going to tell this dude to calm down. Whatever it is, it's going to be okay.' He probably never in a million years expected for it to go like that," said Sharann Long from Upper Darby.
Police said both men were taken to the hospital with non-life-threatening injuries.
As detectives collected evidence at the scene, employees and shoppers were surprised.
"It was crazy," said Spencer Mathews from Cobbs Creek. "We're just trying to conduct business as normal and divert the attention away, but at the same time, this entire parking lot was swarmed with cops and detectives."
"My husband is terrified that something will happen to me," said Long.
Police said the 37-year-old is being charged with simple and aggravated assault.
The 73-year-old victim will face no charges.
Police advise people to not intervene in unsafe situations, but instead call 911.