Sorry to hear of your friend's tragic accident and understand your desire to make something for her.
I would buy the prayer quotes and make them the center of a log cabin block (with only a few logs to get it to the right size). I have no idea what size to make it. You might want to choose two colors that are bright and cherry and make the bottom and left side logs in one color and the top and right side in the other.
In my opinion, a signature quilt would not make a good prayer quilt. A signature quilt is a keepsake and would be hung on the wall instead of being used for comfort. Possibly another friend could gather people's well wishes into a booklet. This would allow her to read their thoughts and prayers while she is cuddling in her prayer quilt.