I can't speak to the risks (and I know that's the question), but we just went through this ourselves 6 months ago, with an 8 hour flight for a relative who had previously suffered a stroke and was undergoing chemotherapy for lung cancer. I think that the best thing we did is put him into business class to make the trip much more comfortable, and he didn't seem to have any particular problem come up as a result of the flight. All the best in this difficult time.