Sounds like a lot of work for 25%. Have you thought about capturing lead data and marketing other stuff to them as well as selling the ebook?
He'd be getting a lot more than 25%. If you do a lot of volume yourself affiliates will come, not to mention if you actually go out and do some recruiting. You could add small upsells or high priced backend products. You could build a customer list you could promote related products. A LOT more jv opportunities when you own the product. There's a lot of other benefits that add up as well. Holding sales during Christmas, Black Friday, etc.
And if you're in a somewhat small niche you'd have a better ROI then all the competing affiliates so you could afford to pay more and still profit, so it makes scaling easier.
And most importantly, it would be an asset so you could sell the business. I've seen many clickbank products sell in the six figure range.
As far as writing the book... that can be a little tricky. There's nothing wrong with copying a simple info product business model, but you got to make sure everything is written unique. Sales page, ebook, etc.