Can you give me some more details
How did you copy them to the external drive?
Do you remember the name of the folder that they were copied to?
Do you think that you might have misplaced them on the drive and put them in a different folder than you thought?
Have you tried searching the drive for specific filenames or do you not remember the names (I know sometimes they are just random numbers or dates)?
How were they removed from your mothers drive (was Windows 7 reinstalled from a disk or did you do the factory restore)?
Have you written anything else to the drive since this happened (don't write any more as that may reduce your chances of recovery)?
Has your mother done much with her computer since the reinstall (don't write any more as that may reduce your chances of recovery)?
Answer these for me and I can probably give you some things to try that are free.