In my opinion, if someone is terminally ill and have accepted and dealt with the fact then everyone around them should try and be supportive of them in what they would like to do before they go, and save the grief for the funeral.
A positive attitude is likely to encourage the person in question, and give them more help and show how loved they are. I also belive a lot of positive attitude can have a surprising healing quality, some people have managed to get through cancer from the worst of situations, is amazing what the mind and body can do. we only utilise a small part of our mind normally- maybe the rest is for stuff like this, i don't know, but just don't blame yourself for anything- you can't change how you feel.
Oli