I use my treadle Singer because it will sew through anything! I've used it on blue jeans, leather, tent fabric, upholstery fabric, plastic screen....you name it, I've sewn it. Since it is mechanical and powered by me there is not much that can go wrong with it.
I am sure there are newer machines that will take a beating and keep ongoing but this is one I inherited so it didn't cost me anything. I tried my new fangled, expensive computerized Bernina machine on a blue jean welt and the fabric pulled the needle right out of the needle holder. That was enough for me to switch to my old faithful before I damaged my Bernina.
The best advice I can give you is to take a piece of the jean fabrics into a sewing machine shop and see how the machines do. As I said, if she is sewing regular sewing fabric and is just looking for a machine to handle the bulk of quilting a big quilt, you want a machine with the largest harp size you can find. The harp size refers to the distance between the needle the motor side of the machine. The larger the size, the easier it is to stuff a quilt through.