Grandmaison said the gunman was lucky to still be alive after the attempted robbery as one of his friend's in the store had a gun on him.
"What you can see in the video is my guy had his gun pointed at the guy's head," he said.
"That's the thing that I'm really thankful about … is just that this guy didn't get killed because it was really, really close to happening."
Grandmaison added that the attacker "got the crap beat out of him really bad" before police arrived.
A spokesman for the Los Angeles Police Department told The Independent that the attack definitely wasn't a prank and the attacker's weapon turned out to be a prop gun.
"The suspect approached the victim with a gun, while a lifestream was in process," the spokesman said.
"The suspect was taken to the ground and the police department was called. The suspect was treated and then booked for attempted robbery."