Feline is always a wealth of information for all of us!
This is a fantastic thing that you are doing and I agree with the rest that she needs to be involved with it.
Also, have her have a look at mid arms. She may not even know they exist, but they are basically sewing machines made specifically for quilting on with a much bigger area than a domestic machine. If she is happy piecing (putting the quilt top together) on her current model but would like a bigger area to quilt in, that might be the way to go. If she already has a lot of rulers for a domestic, they may not work on a mid arm.
There also websites that will rate sewing machines and give you a break down of features, as well, if she's happy with a new domestic machine.
Watson