Bassfisher6522 - That won't do him any good since he can't run the executable on his Mac to be able to get to the point of creating the ISO.
Ejohnston - I suppose you don't have access to a Windows computer where you could run the executable and create the ISO? Maybe a friend or relative has a Windows computer that you could use?
You should have been told to purchase Win 8.1 that included the physical disk instead of a download link. I doubt there is anyway that MS would refund what you paid for the downloaded .exe so you could then purchase a copy with the disk.
I can't guarantee this will work but you might try using Wine on your Mac to setup an environment on your Mac that will allow you to run that .exe and create the...