My concern about a ruffle would be that the fabric, not being quilted, might need to be pressed after each washing, and it would wear out much faster than the rest of the quilt. I would probably tell her to accept this quilt just as it is, and love it as one of her children. (I'm sure none of them came out exactly right either, but she loves them anyway.) Or give it away. (LOL, thinking about the children again.) Make another quilt that has that extra few inches. If she's a quilter she will be making another quilt anyway.
If that's just not an option, then it would depend on the design. You could cut away the binding (assuming it's too difficult to frog it off) and join new quilted pieces all around as if quilting in sections.