How can I recover a Mac-formatted external hard drive?
Windows prompted me to specify which type of file system it was...


Windows disk management console doesn't ask you to tell it what type of file system it, it asks you what file system you want to write to it.  


  Is there any way I can get this drive working on a Mac again?



Don't attempt to write anything to the disk
Duplicate the drive to an image on her Mac (it doesn't matter if it's readable or not) using the following command where X is the drive identifier of your source (corrupted) drive. 

  $sudo dd if=/dev/rdiskX of=~/Desktop/USB_Drive.img bs=1m

Using a tool like Disk Drill or Disk Warrior, see if data can be recovered.  It's free to diagnose, but you'll have to purchase a license to actually recover the data.


You don't have to make an image - that step is optional.  However, if you attempt to make changes and it fails, you still have the original from which to make another image and try again.