Thanks for all the input - I knew you would help me think this through!
I decided that I'm going to use Quilter's Dream Request cotton batting. I think it would be better to use a very thin batting; she can always add another blanket if for some reason it's not warm enough (they did have some highly unusual snow storms and cold weather this winter) but if it's too warm, then she would probably not use it and that would break my heart.
I don't make my quilts for lookin' at - I make 'em for snuggling under!
About the recommendations for silk batting - I think I would like to try that at some point, but not in a quilt that I'm giving away, until I know how it acts. It does sound very luxurious and nice!