B&N uses a DRM method that includes the buyer's credit card number. That is pretty slick, since most buyers wouldn't want to share that.
But encryption and DRM are not foolproof and users will find ways to share if they want to. IMO, your client should think more about his process for selling. For example, if he wants to sell to students, he should make the ebook price part of the class price, which would reduce any incentive to sharing the book. If he wants to charge a really high price for the ebook - which will also lead to the desire to share/borrow - he should reconsider pricing strategy..... people are willing to pay for items if they don't think they are getting ripped off. If he still wants a high price for his work, spllit the book into multiple volumes so the cumulative price gets closer to what he wants.
That's just my 2 cents. Also some food for thought: how much money would JK Rowling have made from ebook versions of the Harry Potter series if she had released them and priced them effectively? I think we all know that people would have bought retail versions of the ebooks..... but since retail versions don't exist, she has not made a cent from them.