Originally, a summer quilt was just a top and a bottom with no filler at all. They most often were tied. To me, any filler might be too warm.

I don't have an answer to your question but just wanted to say that I lived in Mandan for 30 yrs. and always slept with my window opened a bit in the cold, cold winters there. I liked to be covered and warm but loved breathing that clear, cold air up there. I miss ND.

I have a top that should be a summer quilt because I found it in Texas and when I get around to finishing it I will put flannel in it. To me I think a quilt needs something in the middle for body. I do think they were originally intended to be just the 2 layers of fabric, but I might as well just use the sheet if I wanted that.

A very interesting vintage quilt showed up on my Pinterest feed today. Scrappy fish were appliqued to a cotton gauze base. It must have been double gauze from Japan. I've never seen one like it. It got me thinking about making a summer quilt from linen yardage. The applique work would minimize the seams required from using 54" fabric.

I made 2 summer quilts. 1 I did not put anything in the middle and found that it was too "flimsy" and wrinkled easily. The other I put flannel in the center and think that it is just as heavy as using regular batting. I found no difference in the drape/coolness/heaviness. Also the flannel cost much more that the batting that I use. (Hobbs 80/20 cost $12 at HL) Even on sale flannel can be $3./yd and you may need 6-8 yds depending on the size. The next "summer "Quilt that I made iI used regular batting. I keep quilt on all summer and just do not cover up with it.

I've made three summer quilts (or counterpanes as they are also known as). I do not put batting in them. I machine quilted them. The one that has stippling with about 1" spaces I like the best. The other one I stitched in the ditch and the quilting was about 3" apart. It doesn't lay as flat as I like. I love these kind of quilts. They are light enough for summer use. I also use them in the winter when I may need more than one quilt, but less than two on top of me.

The first Summer Quilt I made I didn't put any sort of batting in it. It is fine; we enjoy sleeping under it.
The girl who quilted it said it would have been easier to quilt had there been some sort of batting. She said that with no batting, it wanted to slide around.
The one I am working on now I am putting a flannel sheet for batting. With the back using Flannel, it will have a little warmth for Winter, and I hope it isn't too hot for Summer! Mariah

I would think if the flannel is 100 per cent cotton it will be cool as it will breathe, so will wool. I would be more concerned with the drape and ease to quilt, but that is a personal preference. (My DD's wrap up in my quilts even when it is 80 degrees, and we do not have air conditioning.) I like a bit more body to the quilt. (When I tie my quilts I use the high loft batting.)

I'm very hot-natured but I like a quilt to snuggle under at night when I've got the A/C at 70 ;) it gets pretty hot here but I still prefer the "regular" weight battings

I would either buy her a pretty top sheet or use a thin batting. I have never made a quilt without a batting.