First of all, copyright infringement is the tort, not plagiarism. There is a difference between having a copyright (which, in the US, subsists from the moment of creation) and registering a copyright (which is required under US law to pursue a claim of copyright infringement in our court system.) Just because you didn't register your copyright doesn't mean anyone can grab your published work (and the Internet is a venue for publishing, not the wild frontier) and use it themselves without compensation. The Internet isn't free and while this idiot publisher would like you to believe her stupid, self-interested proclamations, she is wrong, wrong, wrong. She also should have been grateful that you offered a reasonable resolution instead of pissing all over you.
As an attorney, although we are not establishing a client-lawyer relationship here, I would suggest you immediately register the copyright to the article. Since the article was originally published 5 years ago and the infringement has already taken place, you cannot avail yourself of statutory damages or attorneys' fees in your demands or any potential law suit. And I guarantee the runt knows the amount of recovery would be too small to interest any lawyer or make it cost-effective for you to hire one. However, you can probably go to your local volunteer lawyers for the arts and get some help at pushing this along to a reasonable solution.
OTOH, you do have the option of sending a cease and desist letter under the DMCA to any server carrying the material and demand the take down. You find out who the DMCA agent for notice is by going to www.copyright.gov. The letter must contain a description of the work sufficient to identify it, a claim that you are the owner of the copyright, a statement that no right to use the work was granted, and a demand that it be expeditiously removed.
Publicizing what has happened to you is a quick way to humiliate the perpetrator, but it could give rise to a claim of "trade defamation" against you. Truth is a defense in defamation, but it can be expensive to defend.
I hate when these things happen, but as a photographer (yes, I am both a lawyer and a professional photographer) I do recognize that things I put up on line are likely to be stolen no matter how I embed copyright notices or statements that reuse requires a license. Registering the copyright to any work that goes on-line within 90 days of its first publication there establishes my keys to the courthouse door and reserves statutory damages (rather than actual damages) and attorneys fees as compensation for the tort.
Find a local copyright lawyer and plan your next move. You were wronged. She's a thief.