Destiny Faiston, host of the party, says her friend was jumping on the bed when she tripped and fell out of a third-story window.
"We were jumping on the bed and I guess the way she jumped made her fall, and then she tripped and fell out of the window from my bedroom," Faiston said.
The girl fell about 30 feet, and landed on the home's concrete driveway.
"It was very traumatic, I cried a lot," Faiston said. "It's hard knowing that one of your closest and best friends gets hurt badly."
Investigators say the victim is between 12 to 15 years of age, and was transported to Harborview.
Her condition has been changed from life-threatening to serious as of Saturday afternoon.