Better still, Rowling knew who he was. She gave his site, The Harry Potter Lexicon, an award and confessed that she occasionally used its online encyclopedia as a reference. Warner Bros. invited him onto the set of "Harry Potter and the Order of the Phoenix." He even made it on to the DVD, appearing in a documentary included as a special feature.
But all that changed after a little-known publishing company, RDR Books, announced it would release a print version of the lexicon. The author and Warner Bros. sued, asking a judge to block publication on the grounds that it violated copyright law, and the case went to trial this week.
The dispute has thrust Vander Ark into the middle of a closely watched case that illustrates the muddled state of copyright law enforcement when it comes to the Web.
Computers have given just about everyone the ability to copy sections of books, movies or songs and whip them into something new that they can post on the Internet.
The Web is awash with fan-produced material that could be the subject of a copyright fight, from remixed pop songs, to new fiction based on existing characters from books and TV shows, to countless tribute videos cut together with clips from TV shows or films.
"There is almost a parallel universe," said Alan Behr, an intellectual property lawyer in New York. "On the Internet, people basically do things you would never do in print."
And, for the most part, Behr said, the big media companies that own the material being mashed up and manipulated let it slide. There are simply too many offenders to chase, he said.
Warner Bros. and Rowling took a different approach when they sued on Halloween last year. During a three-day trial that concluded Wednesday, Rowling savaged Vander Ark as a plagiarist and a thief, saying the lexicon ripped off too much material from her books. It all reduced Vander Ark to tears at one point during the trial.
It was a surprising departure for Rowling, who has encouraged so-called "fan fiction" and once said there is nothing wrong with people writing new stories for her characters, to share with friends.
The author and her lawyers said they were stirred to action by the proposal to move the Potter lexicon from the anything-goes Web, where it was available for free, into book form, where it would compete directly with a Potter encyclopedia that Rowling plans to write herself.
In short, by deciding to sell his material, Vander Ark was stepping across a line. He was no longer just an enthusiastic fan, but a professional and potential competitor - fair game for the lawyers.
The question now for the courts is whether the lexicon itself violates copyright law, and the decision may not be easy.
U.S. rules allow for the "fair use" of copyrighted material in unauthorized works, but there are limits. Journalists may quote from films and books when writing a review. Scholars can use excerpts from a novel while penning an author's biography.
Generally, the call on whether such uses are legal comes down to how much material was taken and how different the end product is from the original work.
Lawrence Pulgram, an intellectual property lawyer who represented Napster in a copyright fight with the rock band Metallica, said deciding where to draw the line is rarely easy.
"Fair use is the most erratically applied doctrine in copyright," he said.
Works like Vander Ark's lexicon fall into one of the tougher categories. It takes the form of an A-to-Z list of the hundreds of characters and place names from her books, followed by brief entries summarizing how they fit into the plot. There is also information on the origin of some of her characters in mythology and folklore.
Rowling and her legal team acknowledged that readers' guides like the lexicon are, in fact, allowed under the law, but made the case that Vander Ark simply took too much material.
U.S. District Judge Robert Patterson Jr. indicated that the case could go either way and encouraged both sides to settle. He suggested that a creative negotiation might produce a book that both sides could live with.
Rowling said during her testimony that Vander Ark could still do his book, as long as he changed it to take less of her material.
"I never ever once wanted to stop Mr. Vander Ark from doing his own guide. Never ever," she said, before asking the judge again to block it in its current form.
RDR Books Publisher Roger Rapoport said he was open to the idea of revisions but said that neither Warner Bros. or Rowling have indicated a willingness to compromise.
He also questioned whether there was a danger in granting artists too much control over books about them. Should an author, he asked, be able to kill any unauthorized biography, simply because they don't like it?
"We would have to get approval before we could write or publish on people's work. They would control critical commentary on their work, at any time, whether it is our kind of book or an Associated Press article," Rapoport said. "It would create total chaos in the area of critical commentary. Frankly, I don't think that would be good for anyone, even the authors themselves."
Patterson is not expected to make a decision in the case for at least a month.