Keith,
Remember to have her format a disk or two on the IIGS side. The exact drive she is using on the IIGS will determine the level of success with any one disk.
For example, this website reference:
http://www.mandrake.demon.co.uk/Apple/drives.html gives the following information:
"Apple FDHD External (G7287
The FDHD External uses a similar case to the 3.5" External but the 800K mechanism has been upgraded to a 1.4M mechanism. With appropriate software, it is capable of reading PC format disks in both the Mac and Apple IIgs.
It was designed for use with Macs with an FDHD disk controller (eg SE FDHD, SE/30, IIx, IIci). If used with a Mac that doesn't have an FDHD controller, it will operate as an 800K drive. It should not be used with the Mac 128 or 512K.
The FDHD can also be used with Apple IIs fitted with a "SuperDrive controller" or a suitable third party disk controller. If used with a IIgs that doesn't have a "SuperDrive controller", it will operate as an 800K drive."
So, The IIGS might read the 1.4 disk and it might not, depending on her drive. That is why details are so important. Much of the information posted here is general in nature. Trial and error is also a factor when any one drive is having issues. A disk formatted on her IIGS has a much greater chance of being read by dozens of Macs than a disk formatted in a Mac and then taken to the IIGS. It is worth doing both.
Good luck.
Jim