Hi and welcome to the board. There are many things here to consider and I am sure that many people will help me here with suggestions about machines. I will start by asking what does she intend to do with this machine. Will she be only piecing and sending her quilts out to be quilted or will she be doing the quilting as well. I ask because a few years back my sister got my mother a $500.00 sewing machine to replace her old Singer (electric and with foot pedal). My mother never took the machine out of the box. I sent her $$ to get fabrics, because I live in another country and instead of getting fabrics she got her old machine serviced and still uses it. She liked it too much to even consider another machine.
To aswer your question, the speed control is a feature of the machine and some come with a button that adjust the speed, and many older ones had the speed controlled by how hard you press on the pedal.
I am sure there will be many more answers.
Maria