Definitely talk to her PD about her fitness for flying, they can prescribe her supplemental oxygen for the flight and rescue use in Denver. I second the wheelchair. I am flying there next month and when I spoke to the booking agent she insisted I have one at departure and arrival. Denver is a major adjustment. I did my undergrad in Boulder and have had Asthma all of my life, you feel the altitude.
Also, note that every airline has different requirements for traveling with O2 and medical equipment. Maxor sent me luggage tags that identify my medical carry on and it's recommended to get to the airport 3 hrs prior to flight. Some airlines require you to carry 2x the travel time's worth of O2 in case of delays and require a copy of your prescription. United told me to have a paper copy of every medication I travel with, as well as my medication in their original containers with Rx labels attached.