Please look up the word "outlier." Your example of two is trumped by the
example of hundreds , if not more. And since we are giving anecdotal examples of two, my mother had a hard time at 5k ft at first. Another friend from New York absolutely must be acclimated in Denver for a few days. And even then, much like your wife, has issues higher than about 9k or so.
But anecdotes are crap.
Here's what Dr. Ben Honigman, has to say:
25 to 30 percent of visitors heading to the mountains get acute mountain sickness. The risk is lower for trips to Denver, where only 8 to 10 percent visitors get the ailment.
The Altitude Research Center suggests starting by taking a day at a modest altitude that’s between 5,000 and 6,000 feet, such as Denver. Honigman said people who stay a night in Denver reduce their chances of getting altitude sickness by anywhere from 25 to 50 percent. Spend two nights in Denver and chances are good travelers will not experience altitude sickness.
(emphasis mine)
The doctor who studies altitude sickness apparently disagrees.