Give them their money back if you don't want to get a bad reputation.
Also, what format is your ebook? If I was buying an ebook, I would not want a PDF. A PDF is not really readable on an ereader or a phone because PDF's cannot reflow their lines to fit on a small screen, the user cannot control the font size, etc.
I also would not want an EPUB format ebook because I own a Kindle which cannot read EPUBs. If I bought an ebook on a random site, I would want it to be offered as PDF (for those who will read it on a computer), and MOBI and EPUB so I could choose the format my ereader device accepts.
Also, if someone is claiming something is an EBook, I would not expect something the length of a pamphlet.
IMO - Give their money back and make it clear what format(s) of ebook are available and how many words it is.