This is actually a 2 part question:
#1 This person has their own medical/mental issues and cannot make travel arrangements or conduct travel alone. They have not been officially medically diagnosed with dementia because they deny they have anything amiss other than morning headaches and sore throats (from drinking from @noon to whenever they pass out--still legal to be drunk in one's own home.) As the person lives 1-2000 miles from nearest family members, having someone travel to the person's location and then bring the person to family home/funeral location would not be feasible (a previous attempt to companion travel with this person was just short of disastrous, and it was fortunate that TSA did not totally ban the person); travel by car or rail similarly not feasible.
When the parent passes, how/when could/should we tell the person?
#2 The person also resents/fights/obstructs any and all attempts to help them in their own home (given that they deny there are any health problems at all when they can barely walk/drink in excess, they likely also have anosognosia), yet can conduct an impressive 'showtimers' where an unwitting person might think while they seem a bit old-age dotty, the person is essentially functioning ok. They will not accept a cleaning person to help them, nor a visiting home health aide or meals on wheels where someone could check on them (because 'they're not old!!").
They refuse any kind of counseling (because 'those kids don't have any idea what (the person) is going through--they're too young to know anything' (the grief counselors/clinical psychologists who have tried to help are in their 30-40s).
Having a family member travel 1-2000 miles to move in with them/pop in daily/regularly or travel to them is not an option for different reasons (still working/their own health issues).
And before someone suggests exercising POA or getting guardianship/conservatorship, those options have been explored and are no-gos: the latter is impossible to begin with and the person, again, is extremely obstructive/combative about receiving help (even when they specifically request it, sigh.) Regarding applying the POA-- as the person denies anything is wrong/they need help, they won't verbalize that the POA can act for them. AUGH.
At this point, it feels like we have to wait until the person has a serious enough accident at home or elsewhere (they've fallen several times, just bruises/scrapes thus far) to the extent where hospital stay would be mandatory and lengthy/where full dementia/psychological/blood tests could be conducted and the person could not simply refuse treatment/sign themselves out.
If you have tried everything then let them live the way they want. Somebpeople you can't save. You could call APS near him for a wellness check and your suspicion of having Dementia. Let the State take over his care.
In fact when my brother died it was his wish (as it is mine) to have no services and immediate cremation and this is what was done.
There would be little to come out of the information if everything is over.
As to the rest of it, it sounds like the family is amidst a worst nightmare, and I am so sorry for this recent loss and the dire circumstances of the living family member. I would ask wellness checks, APS, and anything else were I living nearby, but I don't know that would help. I am so sorry.
But regardless, he/she should be told when the rest of the family is told as it is still their parent.
What they'd decide to do with the news would of course be up to them. And of course the family doesn't have to step up to offer them any assistance with their travel if they decide they want to come to the funeral. That would all be on them.
Wishing your family the very best as you deal with your family issues.