TLDR at the bottom.
Odd situational question for everyone (or maybe not) since this is out of scope for me. Hoping to get a clearer insight from a more experienced crowd about research, plagiarism and copyright. Unfortunately, I'll have to be vague about specifics and names. Apologies for the long post but I hope someone has some insight.
Just for background information but the theory has been around for almost 100 years but different concepts/ideas are used to prove or further research the theory. I'll start the timeline in the year 2000 just for convenience but information is easily accessible at this time.
Researcher A uses proven concepts that have been used in different fields of study but not widely used specifically in their field however it's not known whether or not they're the first to use. The concept used by Researcher A was discussed on a community forum in 1996 (4 years prior) and on another forum in 1998 (2 years prior) to researcher A starting their own initiatives in 2000. A also made their concept and findings publicly available on their website and started self-publishing a book in 2004 in relation to their findings, 4 years after they first started.
Researcher B is new to the field and comes across the concept from researcher A via their website around 2005 and becomes interested in the concept. During this time they're also reading other researchers' findings (100-year-old theory so lots to catch up on). The more B learns the more they find that while the concept may be feasible, researcher A's findings, definitions of the theory and methodology are flawed so B stops referring to A's research a few months later and starts to conduct their own research. In order to do this, B builds up the concept's methodology from what they believe should work after months of research. Because of the flaws that they found with A, B never read the book that A made either.
After B conducted their own research and findings they also release the information publicly and have changed the methodology of the concept. It's shown that both A and B while using the same theory and similar concept have different results and definitions for the theory. A also congratulates and supports B during this time and admits that their findings and methodology are different from B's. After 3 years of research (2008 now), B decides to publish their findings in a book. While writing the book they never referred to or looked at A's findings but did cite any work that they did look at when writing. A is now claiming that B has plagiarized and infringed on their copyright because of 2 of 6 the definitions written in the book have similar passages referring to the concept; the concept itself is somewhat limited as it's limited to 2 variations for each aspect (so if there are 3 aspects there are 6 definitions). However, these definitions which are similarily written are used for different aspects of the concept which results in people achieving different results for the theory more than half the time. Also since it's a limited scope there's only so much you can write that differs from others' writing.
As far as I'm aware you can't copyright an idea which is the concept in this case and even more so since it's been around even before A started. However, I'm not too sure about the plagiarism. A is demanding to be acknowledged for their work however they didn't contribute to B's research nor were they the originators of the core concept. They just happened to popularize it a little and were B's introduction to the concept.
Has B plagiarized A? Does A deserve the acknowledgement for the aspect definitions despite them being different and resulting in different results? Or just acknowledgement for getting B started on this concept? Or none at all?
TLDR: A accuses B of plagiarism and copyright infringement. B was introduced by A to the concept/idea but never used their information due to inaccuracy. Concept/idea has been around before A started just not as popularized.