I agree with the others. I'm still fighting the battle and making plans for trips. I even already went on a trip a few weeks ago to visit some family across the country. Th hardest part is the airplane ride, the energy it takes to get to and from the gates in the airport, and knowing what you can and can't do energy-wise and health-wise on vacation.
When we visited Vancouver for the day, we went into a public market on Granville Island and I was walking around realizing how crowded it was and let them know i needed to leave, too many people, too close of contact... etc.
Wouldn't you be more concerned if your mom stopped planning things an started thinking there wasn't any use in planning for a future? I've been on the other end of things when my mom was told she had stage IV kidney cancer and she gave up.. didn't even fight for one moment. I would rather have her here today with me and had her disappointed that she had to POSTPONE a trip that she had hoped to be able to take.
Support her, lover her, help her plan it! Plan to go with her if you can.