I am not familiar with that specific machine, so not all of my suggestions may apply.
First thing to check is the stitch length on the machine. If by some chance stitch length is on 0, the feed dogs won't move.
If she wants to try to fix it herself, she can download a service manual for this machine for about $10 here:
https://www.tradebit.com/filedetail....ual-and-owners
This is a good machine and well worth getting serviced. As PaperPrincess mentioned, I would contact a Pfaff dealer to see about getting an estimate.