Follow the directions here:
http://manual.calibre-ebook.com/develop.html You will need to update the correct source file in your code checkout.
Far more convenient than patching compiled python files in the zipped-up resources.
That fix should be compatible with any version of calibre going back at least
to version 1.48.
You might also drop them a line in that forum, that they may wish to use a git checkout of the source code -- advantage is, you can commit the change, then do a "git pull --rebase" every time calibre is released. No need to manually patch the source every time!
Only relevant if you actually can update calibre on your OS...
Enclosed is a copy of calibre v1.48.0:src/calibre/ebooks/metadata/opf2.py patched with that fix.