I have several of Henderson's books but I don't have the one you refer to so I can't verify your statement myself. Just a few thoughts though...
As a sometime autor myself, one of the things I have to struggle with is the fact that almost everything that needs to be written about SQL Server already has been written in some form or another. Between Books Online and the myriad of third party books including excellent titles by people like Kalen Delaney, Ken Henderson, Joe Celko, Chris Date etc... I can hardly think of any areas that are not pretty well covered. Almost anything I can say is merely a rephrasing of what has already been said.
I made a concious decision to wade in (at least up to my knees) and attempt to bring something of value to the community. I applaud others that have made the same decision even if our efforts might be somewhat deficient owing to the fact that I am not a professional writer and do not have the benefit of editors and proofreaders etc...
That said, there is certainly a world of difference between using ideas that have been expressed elsewhere and expressing them in your own language (hopefully recombining them and adding clarity and/or emphasis) and ripping off the work of another person. Ideas and concepts can't be patented or copyrighted, but the process or syntax of expressing those ideas can.
Unless the article is truly a word for word copy (or nearly so) of Henderson's work, Kalpesh Thaker ought to be given the benefit of the doubt.
If most people are not willing to see the difficulty, this is mainly because, consciously or unconsciously, they assume that it will be they who will settle these questions for the others, and because they are convinced of their own capacity to do this. -Friedrich August von Hayek