If you are willing to follow a 3rd party link, the instructions given by this post (blizz forums, but link is in it) helped my wife get passed it:
The TLDR: End the BNET task in task manager, go to hidden ProgramData folder in C: (have to show hidden items) and delete the BNET folder there. This clears the update cache for BNET. For some people, just ending the task works and doesn’t require the cache delete.