Welcome to the forums.
Your post is a bit confusing to me, but let me see if I understand it.
1. Your friend has a 1Tb drive connected by USB to a windows machine and put 15GB of pictures on it in a folder or folders.
2. It was corrupted and she could not open or see the folders in windows, but the drive mounted and she was able to see that 15GB was still used.
3. She moved the drive to Mac, it mounted, and she used Disk Utility/First Aid to try to fix the problem.
4. Now the drive shows only 300GB used (or is that left, it's not clear what you are talking about).
5. The folders are now labeled as Unix Executable files, showing 0 bytes (how do you know they are THE folders?)
If that restatement is right, what format is the drive? NTFS? HFS+? Something else? If it's NTFS, how did she fix it in OS X since OS X doesn't natively write to NTFS drives? If it's HFS+ or some other OS X format, how did she write to it on Windows, given Windows doesn't natively support OS X formats? What Mac, what version OS X? What version Windows? Did she try anything to recover the drive on Windows? Does she have any backups on either machine? Did she get any messages from the attempt to repair on OS X?