And if you can't (or don't want to) open your content server to the 'net, there's browser addons who can display epubs, with no additional software package (epubreader for Firefox, and probably more for other browsers).
And if they don't want to install anything at all, there's sites that can display epubs (magicscroll.net, etc).
Or you can just use Calibre to convert the ePub to .html and send them that ^_^
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