when you sew a narrow seam first, wrong sides together, then turn it to sew again, right sides together, you have made a French Seam.
when you sew wrong sides together, trim away half the seam allowance on one side, fold the other side over and under, then topstitch (as in jeans) it's called a Flat Felled seam.
for pre-quilted fabric i would recommend the Flat Felled method. take the extra step of VERY CAREFULLY picking out the quilting stitches and clipping out the batting from at least one side of the seam allowance. preferably both.
since it's for a bedspread, i'd suggest another option: use a very narrow sashing strip on the back so the two edges touch when it's flat. then attach another sashing strip - any width she thinks would look good - to the top. like a quilt-as-you-go might be done.