Q. A friend of mine was just diagnosed with cancer. I want to pray for her so she gets better. But how do I know that’s really God’s will?
A. Your question is a wonderful question to ask! And the fact that you ask it reveals that you already understand a lot about the "mystery" of human illness and suffering.
When we are confronted by a serious illness, whether it is our own or a loved one, we experience many emotions, feelings, thoughts and questions. We may find ourselves, at first, in denial, especially if the illness is serious like cancer. We may may experience confusion, despair, sadness, anxiety and anger. We may immediately look for a way to be cured and turn to God for our healing. All of these experiences, thoughts and feelings are very normal and fully understandable. But, in the end, I think your question gets at something very important: "What should we pray for?"
The best advice I can give you is what the Catechism of the Catholic Church teaches. One area we find answers is in the teaching on the Sacrament of the Anointing of the Sick. First, regarding serious illness, the Catechism states: "Illness and suffering have always been among the gravest problems confronted in human life. In illness, man experiences his powerlessness, his limitations, and his finitude (C.C.C 1500). It goes on to say, "Illness can lead to anguish, self-absorption, sometimes even despair and revolt against God. It can also make a person more mature, helping him discern in his life what is not essential so that he can turn toward that which is. Very often illness provokes a search for God and a return to him" (C.C.C 1501).
So the first thing to point out is that a serious illness, like cancer, will have a dramatic impact on a person and that person's loved ones. But what we must realize is that even though serious illness can be very hard to experience, there are many good things that can come from it. It is, in fact, a very good thing for us to realize that we are ultimately powerless in some areas. This is realization that can help us turn to God and rediscover what is truly important in life. This is a good thing!
When it comes to seeking a cure I'd say that it is essential that we seek the best medical attention we can get. God gave us science and medicine for our good and the more we can use it responsibly the better. So seeking an appropriate medical cure for illness is always a good thing. But, in the end, we need to realize that our lives are ultimately in God's hands. So as far as praying for a cure we can turn to the purpose of the Sacrament of the Anointing of the Sick. The Catechism states that one of the purposes of that Sacrament is "the restoration of health, if it is conducive to the salvation of his soul" (C.C.C 1532). This is a very important statement because it shows us that the most important thing to pray for is not only a physical cure. What's more important is to say something like, "Lord, if a physical healing is what is best for my friends soul then please heal him/her." God is the only one Who knows what is best for us and this kind of prayer respects that fact. This is important because, in the end, the most important thing is that we let an illness simply draw us closer to God. And if that illness is to end in physical death, what's most important is that the person grow closer to God through their illness so that they are ready to be with God forever!
This is hard to deal with in life but in my experience as a priest God gives those who are seriously ill a lot of extra grace to begin seeing things this way.
Perhaps the best prayer of all is simply to surrender your friend to God and tell God that you trust Him and that you want what's best. God can sort out the details and conclusions if we only trust Him!