Have you tried plugging the drive back into the mac and seeing if themac can see and use the drive?
I ask because the normal foramt (NTFS) that windows uses cant be read on by Mac and it mayhave asked her to format the drive to begin using it. The other format externals like to use for compatibility is fat32 which works on both systems.
So IF you can use the drive on her Mac without doing anything then she formatted the drive on you.
Another possibility that happens all the time with external drives is disconnecting them without first using 'safely eject' first (I'm not sure what terminology Mac uses but its there) and this can easily corrupt or even damage the drive.