If he bought the book(s) new originally he wouldn't be "making money off [Lyle's] work", but rather just recouping some of his original expenses. I do agree that putting them out on the secondary market could conceivably eliminate a future sale or two of whatever books he's thinking about selling, but, one could also envision that it might spark additional interest in Lyle's work, which in turn could generate additional website visitors and potential sales of new material. The net effect could go either way.
OP, maybe donate it? If you Google on something like "where to donate used books", all kinds of options appear. I'd think that if you sent the books on their way via one of those routes, they'd at least have a chance of landing in the hands of folks that probably wouldn't have bought them new anyway, which in turn at least provides further exposure to Lyle's work. If this ends up generating even one additional sale of a new book from his site, excellent! If not, well, at the very least it provides the chance to expose someone out there to real, honest-to-God first class information on the subject. Given the sea of drivel that exists out there in this field, I'd find it hard to argue that this wouldn't be a good thing.
Anyway, just thinking aloud.