If it's just quilting, a holder or hoop such as Martelli has with handles could help with that. There are feet for most machines that will guide for piecing.
Even studying such a hoop might give you an idea.
My Pfaff 2170 has what is called a continuous hoop. You can do a length of fabric in multiple hoopings by lining up in the software. I've made yards of edgings for garments.
Since she might not have a machine of that capability, watching these videos might help design a guide that would allow her to straight line quilt, or even meander with the hoop. Most here pooh pooh the hoops, but I have a Martelli with the grips that I used while one hand was in a cast. It worked.