I had a friend staying with me, had never been above 4000 feet in his life, has smoked for 9 years and is not in the best shape. He gets drunk enough to have a hangover. I drive him up loveland pass once the hangover wore off, he decides to start hiking up on the west side. We kept going till we were looking in on loveland, he walks the whole thing like he is at sea level. I guess some people are just lucky.
Every time your friend climbs, will be different...maybe the next time he won't make it. People never know when they'll get altitude sickness...just look at those folks that climb Everest!