I talked to my Aunt, who cross stitches and quilts, and said she mainly has done this for wall hangings. With that said, all she did was zig zag the edges of the Aida cloth to keep it from fraying and then just quilted the fabric that was added to it.
Now, if it were me, I would do the zig zag also, but I would also use some kind of interfacing so that batting would not come through just like "Jan in VA" stated.
I wish you both the best and can not wait to see a finished picture if your Aunt would be so kind to share.
Julie