The vibrant 26-year-old, who had just moved to New York City, was reportedly drunk and trying to get back into her apartment from the fire escape when she lost her footing.
She fell five floors, before slamming into a concrete passageway in the early hours of the morning.
Her neighbours awoke to the sound of her body hitting the floor - and the screams of her horrified roommate.
Speaking with The New York Post, Sangelys Perez said: “Her roommate opened my window.
“She was crying. She said ‘I’m sorry, I’m sorry’.
“I asked her ‘are you OK?’. She looked down and said ‘she fell she fell.’
The 15-year-old added: “I was scared… I didn’t know what to do or say.”
Just one week before her death, Kasey shared a photograph of herself leaning over the very same fire escape from where she would later fall to her death.
The image is particularly haunting, as a friend commented that it made him nervous when she posted "these 'casually leaning over the edge' pics."
She replied: "Well I have a lot of whiskey to help me out."
Jesse Ray Mendez told the newspaper that Kasey and her friends used to drink on the roof regularly - and that he’d warned them of the dangers before.
"I feel guilty,” he said.
“This could have been avoided, and they should have listened to me the first time."
Kasey’s Instagram, which is filled with snapshots of her fun-loving life in NYC, has now been flooded with messages of condolences from around the world.
“Beautiful pictures,” said one. “Rest in peace.”
Another added: “You were a pretty girl who seemed to have a lot to live for. Glad you got to taste some of home before you unfortunately left this earth RIP.”
And, for those trolls who mocked the young barista’s death, or judged her decision to drink on the roof, her friend had an important message.
She wrote: “She'd done it a million times and, though it could have been avoided, nobody lives their life thinking what they're doing will actually cause their death.
“Every single person commenting has had a drink I'm sure and certainly made questionable decisions through life.
“Kasey was a human being that many people loved and cared about.”
She then asked that people stop posting such insensitive comments on her friend’s page, insisting that Kasey was not something "to judge and gawk at".
Our thoughts are with her friends and families at this sad time.