Well, saying "hundreds, maybe even thousands" appears as if he can't really measure what exactly should be paid. There is probably some sort of idea that they made book, he did X copies, and I got Y money, he ripped me off. Without knowing the details of deal, we can't say much.
This has been seen so many times in music business. Band gets deal, expect X% of royalties. If this royalty is not paid in % of physical records, I smell trouble. They'll measure 1000xCD x 12 euro = 12000 euro of money.. where is my several thousands of euros of royalties? And when you really go down to reality of business, in many cases label has not generated income of any relevant level. They probably struggle with several such projects, thinking where this money should be used.
So.. what was the deal? Did artist assume his work is a bestseller? How many copies was made, how it was distributed? Does he know the details of printing costs, shipping, retailer return policies, overall level of prices, etc. Did they work under firm agreed sum of money, or something based on potential profits what was never reached?
Certainly a lot of small publishers are somewhat unreliable. Easiest would be to just honestly tell the breakdown of how it's going to be. But let me think of reality, where guy who could do years of work, get couple of hundred pages book ready and then publisher tells, this is going to sell maybe 100. You get paid royalties when we have covered all the costs and are making profit. And due nature of business, this "profit" ends up being everything else but worth of couple years of work... It's like saying, "lets do book, you get 20 free books for all the work". And author may ask.. 20 books for ALL this work?!? Then you may try to explain that there just ain't "market". Ask him to go write a blog and join modern world...