It appears to me that the original quilter was going to make the quilt by sewing the right sides together on three sides and then didn't get it completed. Then someone else came along and started quilting it on a "guess" that this was correct. I would fold under the the 4th side and hand stitch it. Then complete the quilting if possible - if not take out the beginning of the quilting and redo that. Then, hand or machine quilt it. And if neither of those work, take the completed top apart from the backing, put in newer batting, and add the backing (either original or new) and use the binding that is there to complete the quilt. Make a label with both the original Quilter and the 'fix it" quilter with love for the recipient of the quilt.