If you let your friend use the SD card that was formatted for use in a camera in their computer, then likely it messed up the file system for the card. Or if the friend used it in a camera that used a different file system, it would also have messed up the card. I am not saying the photos are un-recoverable, but most likely you would need some specialized software, or if you really want to recover the photos, take it to someone who specializes in data recovery. Maybe someone else here has some ideas, but if the directory of the SD card gets messed up, it needs to be restored. That doesn't mean the data (photos) aren't still there, but that the device can't find them.