He could promote the books sale on the web page as a courtesy for the mistake. The author may take the gesture as more economically favorable than a fight would be.
I think this is a great idea. I'm sure the fine print of terms of sale of the eBook clearly prohibited anything like loading it onto a publicly accessible website. But it sounds like numberonepal honestly didn't realize the general public would be able to find and access it there. A free ad with link to the site where it can be purchased would be reasonable compensation.
I'm sure the author DOES very wisely do periodic searches to see if the book has be illegally posted to publicly accessible websites. He'd be crazy not to.