You have a couple of typos to start with.
Script-fu doesn't use underscores in the built-in function names and constants:
gimp_drawable_transform_flip_simple image
and
ORIENTATION_HORIZONTAL
From the presence of "Program Files" in your command line, you're on Windows? Well you can't use single-quotes to enclose the batch command:
"C:\Program Files\GIMP 2\bin\gimp-2.8.exe" -i -b "(batch-flip \"*.png\")" -b "(gimp-quit 0)"
You've also defined the script as taking a variable called "image", but expect it to be called "pattern" when doing the file-glob
Kevin