It's possible that the epub is being stripped in transit, you could start with having your client forward (not reply) the message to you, which always gives the option to forward attachments.
The firmware doesn't have iBooks built in, so it's possible your client never installed the app. Your client could also install Stanza, one of the better alternative epub readers. If your client is still stuck, they could try to download an epub from Gutenberg, like
http://www.gutenberg.org/cache/epub/76/pg76.epub .