I have made these comfort quilts my mission, and have made over 200 of them. For lightness of weight, I only use a fleece backing and no batting.
I try to choose colors and pattern based on what I can find out about the person it is for. As others have said, some do want the colors, ribbons, etc. that signify being a warrior against that disease, but I would only do so if I knew that to be the case. The most successful are those geared to interests of the person... horses for the horse lover, etc. but any quilt made with love is a comfort and treasure.
Sturdiness and ability to endure many launderings is important. I tend to stay away from pastels and lighter colors only because they show stains more. But having said that, if a pastel quilt is what the person would love, then a pastel quilt is what I would make.
Your caring enough to look for information before making the quilt for your friend shows already that your quilt will have the essential elements; love and care.