It's not an xml file. It's a zip file with a bunch of xml and xhtml files inside of it. If you rename the file .zip instead of .epub, you can open it up. But still, to fix it, you will need to know how to tweak html and css and my guess is that the code will be a bit of a mess. Then you will need to zip it back up and rename it epub. If you are on a Mac, you need to use terminal or a script to do this.
The person who converted the pdf to epub via Calibre doesn't sound like a pro to me. Not only is Calibre not the right software for publishing (it's more for personal use) but PDF is probably the worst format for converting to an epub file. I assume, since it was written by your boss, that you have the original source (e.g., Word file, InDesign file, etc.) from which the PDF was created. That would be the ideal starting point. It seems like you may need some help with this. If you don’t want to tackle this yourself, there are many graphic designers on here who specialize in ebooks and can provide assistance. I am one of these and would be happy to offer my services.
My web site is
http://illuminated-pages.com. Take a look. If you like what you see, send me a message and I can give you a quote on your book. If it is straight text, that's pretty easy to convert and not that expensive. If it has a lot of images and tables and such, that will be a bit more but I can't say until I have seen it.