I found the pattern for breast cancer pillow, I think it is the one you are looking for. Please Pm. I have tried to post pattern, but can not get it to work.

From my own experience with a mastectomy, I'd suggest a relaxed V shape. Soft as a baby's pillow. I saw one, think it was here, shaped like a long, skinny heart. That might work too.
That is very nice of you to make something like this. You're the kind of friend everyone should have, and wish for.