In the case of the OP, if the “fully human” women is not interfered with, she will beocme fully spirit and return to God. Which, of course, is the ultimate goal of us all. If she is interefered with by the use of antibiotics, her return to God will be delayed by a few weeks or months.
Well, being the OP, I should try to clarify. Since I am someone who nearly died from Pneumonia, I may be biased in some way. If I believed that she would simply turn from human to spirit and go back to God, I may have reacted differently. But knowing pneumonia, I thought that she would suffer greatly, drown in an ever-increasing gathering of fluid in the lungs, then die and go back to God.
If my opinion, which was to administer normal treatment, seemed cruel or unloving in any way, it may have to do with
my memories of gasping for every breath and drowning from fluid in my lungs.
In the first case it requires an action (abortion) to end the natural cycle. In the second the natural cycle will not be interupted by either an action or an inaction. Only the timing will be effected. The patient is terminal, barring divine intervention in the form of a miracle. And since God has set these things in motion, why should we interfere with how he chooses to end this persons life?
Again, to clarify my thinking when I started this. I did not believe that only the timing would effected. I know that other systems are going to shut down; probably soon. There will be no way to stop that. And there may be a chance she has a peaceful death in her sleep without drowing from pneumonia, gasping for air.
So who am I to decide that is right? Just a son-in-law who is low on the totem pole and never even got to give my opinion! (But we sure did get to explore the subject HERE!

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You seem to believe that we are in sin if we, as family and caregivers to our loved ones, decide not to interefere with the natural course of events; if we choose not to interfere with God’s plan. But this cannot be known unless you can see with the eyes of the Father, into the hearts of those involved.
Again clarifying the original post - Yes, with my basic and incomplete understanding of Catholic thinking, I believed someone would be guilty of grave sin by not treating her. (And, yes, my wife felt the same way )
I surely don’t KNOW for a fact what’s God’s opinion would be on what was grave sin and what would be compassionate "letting go, " in each individual case,
but in this case, my opinion was to go for the ordinary treatment.
I don’t know what happens next, and I do believe you are right, that this treatment only delayed the inevitable death which is coming, probably soon. My opinion, which could be wrong in many people’s eyes, is that avoiding death via pneumonia was an act of kindness and the right thing in this case.
I’ve heard the old slogan, “pneumonia is the old person’s friend” but I think that is a strange expression and it is no one’s friend. a car accident with an instant death could be a friend. Drowning on the inside just doesn’t seem friendly to me.
Again - I’m not commenting on any other person’s experience, trials or suffering. I Just came here for advice and hopefully to find a solid “Catholic Answer”