There is a huge philosophical difference in attitude between hospice caregivers and most of the rest of interventional medicine which is aimed at fighting disease with every option possible (including dietary restriction if necessary).
In order for a patient to qualify for hospice care there must be a definite medical diagnosis attesting to the patient not surviving longer than a year (and usually shorter). Therefore the emphasis shifts from cure to palliative care (making the patient as comfortable as possible in whatever time they have left) and as good a death as possible.
When I was a live-in caregiver for an elderly couple, the gentleman was eventually diagnosed with Pancreatic cancer and they gave him about 10 months or so. When hospice became involved they gave us literature and talked to all of us about what their philosophy was and what things took precedence. They made certain that everyone knew that the end was inevitable, there would be no false hope, no last minute heroic medical interventions, no miracles etc.
This is so different from what most people are used to so they made it crystal clear that the total emphasis was on making him as comfortable and happy as possible.
And it makes perfect sense when you think about it logically. There’s not a single thing that your brother could eat or refrain from eating that would make any significant difference one way or another to his lungs. It’s not as if an abstemious diet will regrow lung tissue or something. So, he may as well eat whatever he enjoys.
And obviously that puts him in a much better mood and it seems like he has faced up to the fact of his imminent death and realizes that there is no more that can be done. He may find it less sanguine as time goes on but for now he seems to be at peace about it.