Is the PDF legally available online? The sources I see don't appear to be.
I don't think it is legally available online. Getting the physical book would be the best option since its legal and more thorough than any online resource I've found, but I don't know what your budget is like or if you even like reading books. Just saying that its one option you have.
In about 30 seconds I found several places to get it as an ebook, and at least 3 that have PDFs.
O'Reilly's a good place to buy from.
Or he could just buy it. It's well worth the price.
He asked for an online resource, I gave him one. No shit buying the book itself is better.