I really don't like to cast aspersions... but..
If he dies and they are not there, that gives them something ELSE to rage and rail about. 'We were denied... because they made us fight.. because .... ' etc etc, blame someone else.
I can understand the desire to lay blame somewhere else if one feels horribly guilty (whether that guilt is deserved or not) about actions that may or may not have led up to someones death.
I also think theres a good deal of 'attention' to be had if they refuse to be there in the hopes it will delay switch off (whether they are actively hoping he has a heart attack and dies that way or not) and try to stop the hospital doing it on the grounds that they are not there for insert spurious reasons
One could be more generous minded and consider that his mother having made such a hugely public fuss and attack on the hospital and nurses... might choose not to be there to avoid 'that whole thing' - but then she has a whole army, mostly of her own creation, behind her who would likely revolt at the idea she would choose NOT to be there and thus to 'let it happen'.. so shes never going to admit that...
I really want to feel sympathy for her, and his whole family tbh, but I am struggling given their behaviour.