Resolving authorship when PhD student does most of the work on paper, but originally agreed to support another PhD student (first author)?
I personally find the "* authors contributed equally" footnotes to be supremely awkward, though perhaps you're in a discipline where they are common, in which case go for it....  And in this case you're arguing it isn't even true.  I'd say you're getting a raw deal, but the time to push back was earlier, when you could have argued for the appropriate recognition and then declined the chapter inclusion if you didn't get either the proper recognition or more work from your colleague.  

My advice: treat this as a learning experience, be gracious, and move on.