Why not maintain control of the ebook and have users download it from a co-branded website that you control?
Have the co-brander add a DNS record for, say, ebook.hisdomain.tld, and point that hostname to YOUR website.
The web page(s) that you set-up at ebook.hisdomain.tld would not (unless desired) identify you, and would have a look and feel identical to the co-brander's site.
Or, you could simply host the downloads on this site, with no actual web pages.
In either case, you can use some kind of validation token and/or referrer URL to insure that download requests are sent from the co-brander's site and are authorized (if required) by the co-brander. (Say, if he is charging for the ebook, or gives it as a promotion only with a certain purchase or membership, etc.)
In any case, this gives you the flexibility to charge by the download, or even if you are charging a flat monthly or yearly fee, at least you will now know for certain how many have been downloaded, so that you can adjust your fee in the future.
I know there are also various DRM schemes for ebooks (isn't this supported by PDF?) which might accomplish the same thing, and perhaps more securely, though I'm unfamiliar with the details.