"We got stuck on a rock and, so, my Papa tried to floor (the) Jeep to get it over the rock, and the rock slid out and we just rolled down the hill," Austyn told KFOR in Oklahoma City.
Amanda Kerley said Greenough did all he could to protect his grandson.
"My dad grabbed a hold of him and just cocooned him, just totally protected him from as much as he could as the Jeep rolled down," she told the news station.
Austyn recalled the moments before he was ejected from the Jeep.
"I remember his arms around me, because I held onto his right sleeve all the way down until he let go of me and I lost grip and flew out," the teen said.
When the Jeep came to a rest, Tony Kerley used the winch on his own Jeep to rappel down to where his son and father-in-law lay injured.
With Tyler working the winch to pull them back up, Tony Kerley got Austyn to safety. A couple who happened by and began helping in the rescue took Austyn down the mountain to a rescue crew, who flew him to University of New Mexico Hospital in Albuquerque.
"He had internal bleeding and they had to remove his spleen," Amanda Kerley wrote on Facebook. "He has several broken ribs on the left, a collapsed left lung, lacerations to his pancreas, lacerations on his legs, arm and head. He has fractures in his neck and back, and both clavicles are broken, too."
The teen, who lost more than 40 percent of his blood volume, had to be resuscitated at one point, Amanda Kerley said. Despite the massive amount of physical damage caused by the crash, Austyn survived and has recovered enough that he was allowed to return to Oklahoma last week.
Greenough died at the scene, despite the efforts of his son-in-law and passersby.
"One of them was a paramedic by trade and went down there, and he did 15 to 20 minutes' worth of CPR on my dad, and he just never … my daddy was gone," Amanda Kerley told KFOR. "There's no doubt in my mind my daddy gave his life to save him, and he would do it over and over again. That's who my dad was."
On Facebook, the distraught daughter described her father as a man who loved his family and loved the mountains, where he ventured every year with his son-in-law.
"This year he was beside himself with joy that his two grandsons were coming with him," Amanda Kerley wrote. "He loved them deeply! He loved us all deeply and with great passion.
“His family was his happiness. And it hurts to know he’s gone.”