Death is apart of life. Its not meant to be tampered with by us. There are great healers in this world that fight death everyday (spiritual healers, natural medicine practioners, and doctors) and they all realize that at some point its time to let go. It is best to just keep the person comfortable, maintain a loving environment and let them pass away when death decides to take them.
I understand its hard to watch a loved one suffering, knowing they are going to die, but people die when they embrace their death. People have the choice to fight death, and to some extent it is allowed. But death also has a way of waiting until that person does accept it or at gives them the chance to. I've seen it personally and heard of many personal stories. If you quicken death, you could hurt the person spiritually, emotionally, and mentally by not giving them the chance (that life is giving them) to accept the inevidable. A peaceful death is an accepted one. Suffering is only our bodies way of telling us that we don't have much time left to say the things we want to say, see the people we want to see, come to terms and let go of the life we have lived.
Not everyone has that opportunity. And many that do not, and cannot come to peace with their death, will suffer in the spiritual instead. Do you really want that for your loved one?
Even some coma patients have been proven to still have some understanding of what is going on around them. So no matter what the case may be, your place in this is to give emotional and mental support. A comfort and peace spell is the only thing you should do if they cannot be healed.
You don't want to control death. It can backfire terribly. Those are powers beyond us that know exactly what they are doing. Mess with that and you'll suffer the consequences, or your loved one will.