I would suggest something light - like polyester batting - and floral - maybe a four-patch posy because each square is enjoyable to look at on its own. Calming blues, some pink and/or green, and definitely a little yellow which is said to be good color for the depressed, for what that's worth. I would avoid anything mawkishly sentimental at this point - accentuate the positive.
I'm sorry to hear about the cancer and wish her the best