I do not believe that the custom folder works for simple-border. It wasn't added as of yet.
The reason they (the texture folders) do not work in F drive is because Gimp is looking for the GIMP 2 directory (.gimp-2.8) in the C:/USER folder.
The PATH environment variable is set for C not F.
You could try adding the value for the PATH variable like this at the end of the rest of the string.
F:/PATH TO YOUR /user/.gimp-2.8 folder.
That may work but i am not promising anything.
Alternatively you could add the path to Edit>Preferences>Folders>Patterns This will cause all the images to be added as patterns and you would open one as an image and edit it from there. They would not however be available from Beautify interface widgets.
Have you tried installing Beautify into an F drive gimp install (USER/.gimp-2.8/plugins folder)?
Then just run it once close it and place the texture folders in the .gimp-2.8 folder that should now be available/created. For the life of me i can not remember if Gimp will install that USER sub directory in F drive though.So you may be out of luck there.