Check they're not running IE11 in Enterprise, Compatibility or Quirks mode. Easiest way to do that is to open the developer tools and check it shows '11' as the Document Mode.

Equally we've had instances before where the client's own security systems are stripping JavaScript code from the .zip file as its being uploaded to SF.

I too don't have access to SF, so I'm relying on our client to do the testing for me and provide the results.

Yeah, welcome to my world. Fun, isn't it?