I had lots of trouble with the last Pfaff I bought and ended up getting rid of it. (Changed to Baby Lock and love it.) Pfaff changed ownership a few years ago and unfortunately they are no longer the same brand that was known world-wide for sewing machine craftsmanship. I would not buy another one, nor would I get a Singer or a Viking. (They are all part of the same company.)
Can your mom trade it in for something different? There is nothing more frustrating than a misbehaving machine.