The cable itself is curling or the stitches around it? A lot of cable patterns have purl stitches around them so the cable, which is usually done in knit stitches, stands out. To create the purls you have to use knit stitches on the other side which creates stockinette. Stockinette naturally curls so if that is the problem she'll need to have a border to prevent it. Garter or seed stitch is often used for this.