Definitely need to acclimate! If you make that potential trip to Estes Park at some point, definitely give yourself adjustment time!

Oof, yeah. Especially when you're doing anything super physical, like skiing/climbing/hiking, the altitude can smack you down hard. Yikes that it was bad enough to go the ER for him! I know we had a couple people who had some bad bouts of altitude sickness (though not ER worthy, fortunately) when we did our archeology dig, even just with the change from our norm to the higher country of the site.

That is genuinely not a bad theory... the number of people who make bad or weird choices about things like going off-trail and getting lost, even when they're typically reasonably experienced, or just in general behaving weirdly, even if they don't do anything dangerous. It messes with you!