Actually, I completely trust the UX pilots who fly DEN - ASE, having done that route numerous times a year over 30 years. They are the best and know every square inch of the Rockies.

On the flight to ASE, tell your friend to sit in the AC seats (left side). On departure from ASE, sit in the DF seats (right side). You avoid looking at the hill next to the plane as you land/takeoff, which can be a bit disconcerting to the nervous flyer.

I think the backup airport is usually Grand Junction. Only happened to me once.

Much rather fly to ASE than Telluride. That can be freaky!

FWIW, the only really terrible incident in ASE that I can recall was the one involving a private plane where there were reports of pressure on the pilots by the passengers to land in a bad snowstorm.