The Grandmother I speak of was 100 years old, bordering on 101. She had DOUBLE LUNG pneumonia. She had made it very clear to NEVER go to heroic efforts to save her life. Well the doctors were pressuring us to insert a feeding tube - she fought it. She began with liquid foods and slowly kept it down. She totally recovered and went back on to solid foods. A few months later she gently died of heart failure. God takes people when HE is ready.
Since we humans do not in fact receive regular reports from God about what He wants, we have to make do with applying principles based on reason and Revelation.
Your grandmother was clearly not attempting to commit suicide, and her judgment that the feeding tube was unnecessary turned out to be correct, as shown by the fact that she was able to get nourishment without it. If she had been unable to do so, she would have died of dehydration/starvation without a feeding tube. What the Catholic Church would object to in that case is the hastening of death by the denial of a basic human need, food and water.
The crux of the question is: Is the use of a feeding tube, in itself, a “heroic,” or “extreme” measure. The answer is: No. The conclusion follows that the use of a feeding tube is obligatory unless its use would do no good or will do harm.
The fact that God can, and does, do what He wants is no guide for us humans when it comes to making decisions, because He reveals nothing to us about particular situations. We have only the data of reason and the data of Revelation to enlighten us. It is undoubtedly true that God takes people when He wants. But we have no knowledge of when that is until it happens, and so, obviously, that particular datum cannot possibly figure in the decision-making process.