Plagiarism is taking the work of others and passing it as your own without crediting the authors. In this case the photographer created his work of art/composition, the user's work is completely based on the work of art of the photographer, a derivative work, and she doesnt credit the author, hence, its plagiarism.
The work being in a different art medium is irrelevant, since its clearly a derivative work regardless of the medium.
Additionally, she doesnt have permission to monetize derivative works of that copyrighted photograph, so she is also infringing copyright law. This can easily result in a DMCA takedown notice, and you as a witness should know this.