I think you are confusing plagiarism with copyright infringement. Plagiarism has more to do with professional credibility and originality, less to do with legality. Many people value creativity and originality, they do not like to see the things other have worked very hard to create and promote bastardized and rehashed without their permission. A work that is derivitive is often of little literary or artistic merit on its own, simply being a recreation of the original characters in hackneyed plots and situations in order to siphon off the popularity of the original work.
That he has been inspired from so and so author, then it should not amount to plagiarism. As otherwise, we wouldn't have the wonderful fanfictions. It should not amount to copyright infringement. Fan fiction means the original author has great popularity, which is welcome, isn't it? Who would be unhappy with it?
Something that is inspired by something else is different then someone taking a section of a book or work that is someone else's and publishing it, claiming that it is theirs. For example, if I wrote a fan fic about the Hunger Games and some alternate ending, and wrote entirely out of my own ideas and thoughts, simply using the same names and background stories, but rephrasing it in my own words, it is not plagiarism. Now, on the other hand, if I wrote a fan fic about the Hunger Games and copied pieces of the book into my book and publishing it as my original ideas, that is plagiarism.
In order for a work to be considered plagiarized, the author must try to pass the original work off as his own. Fanfiction carries distinct labels and warnings- what the original work is (i.E. "Harry Potter Fanfiction"), and that the characters are not the author's nor does the author make any money from the work.