The files on the SD card could be accidentally deleted, or she used cut and then paste in which case pictures are erased after move. But do not worry, a sd card data recovery tool should be helpful, here's a good onehttp://goo.gl/4Ghhqa
To increase the chance of SD card file recovery, you'd better stop using the SD card before you get back lost files from SD card. This is because that continue using the SD card will most likely save new files to SD card where those new files could overwrite the space of lost pics which will make them unrecoverable.
There's some other chance where files could be hidden due to unknown reason, so you may also try to show hidden files, just to check whether this will bring up the lost photoshttp://windows.microsoft.com/en-sg/windows/show-hidden-files#show-hidden-files=windows-7
You may also scan the SD card for virus, if it is virus infected, it's better to kill the virus, here's some anti virus software you can usehttp://www.norton.comhttp://www.mcafee.comhttp://free.avg.com