When Is It No Longer A Good Idea To Fly,

When is it no longer a good idea to fly, when suffering from COPD? My MIL is what we believe is stage 4 of COPD (she won't tell us). She is very thin, under 100 lbs. and requires full time oxygen (but won't use it full time). Complains that the hose hurts her nose. She does treatments before leaving home and takes her oxygen with her, but waits until the last moment to use it. This usually causes a panic attack. Some have resulted in strangers calling 911 for her. She needs a wheelchair, walker or electric cart to get around, otherwise she can't walk from her car to her front door before being exhausted. How do we convince her that flying is not safe for her? She wants to fly back to her home state which would be about a 3 hour flight.