I believe that Graham was rescued by Boomers whom either took him to Followers or possibly Nelliis. Modern medical care is the only Graham could have
possibily survived. What if Riley's Rangers had a branch in New Vegas and picked. Graham. up.?
HEre's a thought: NCR forces rescued and operated on Graham. Graham was given a choice: help the NCR or else be shot.
Actually some people have survived incredibly high falls. A woman named http://en.wikipedia.org/wiki/Vesna_Vulovi%C4%87 survived a 33,000 foot fall (of course it was partly being still in the plane that saved her life, but still).
There's also been people who have attempted suicide in my home town, San Francisco, on the Golden Gate Bridge. The Golden Gate's 220 feet above the water, and about 20 people have survived the fall (albeit with a lot of injury). Depending on what part of the canyon he fell from, his physical structure, etc. could save him. He'd be in a lot of pain and have a lot of broken bones, but it doesn't mean he'd be dead.
But this is also a fictional universe. They can say he survived it "just 'cause".