I would first confront the researcher concerned with the allegation and evidence of his misconduct in order to know his reaction. They are either guilty (which is rarely the case today, in my experience) or evasive or even rejecting. You can involve other colleagues in how they assess the situation.
There are at least three ways to react to such an incident: In a personal confrontation and settlement of the conflict. In a scientific dispute: One writes an essay in which the facts are presented - and publishes it. Thirdly, in an institutional way.
For example: There was a recent incident at a southern German university in which an internationally renowned, high-ranking natural scientist, who is already emeritus but continues his research, was proved by another scientist that his published data on a project was not correct. The journal, which was supposed to publish the criticism of the respected researcher, hesitated for a long time to do so. Meanwhile, the university itself set up a commission of inquiry with experts to examine the facts. Apparently, the critic is proved right. This was suggested by a media report. The case is probably not quite closed.
I would first confront the researcher concerned with the allegation and evidence of his misconduct in order to know his reaction. They are either guilty (which is rarely the case today, in my experience) or evasive or even rejecting. You can involve other colleagues in how they assess the situation.
There are at least three ways to react to such an incident: In a personal confrontation and settlement of the conflict. In a scientific dispute: One writes an essay in which the facts are presented - and publishes it. Thirdly, in an institutional way.
For example: There was a recent incident at a southern German university in which an internationally renowned, high-ranking natural scientist, who is already emeritus but continues his research, was proved by another scientist that his published data on a project was not correct. The journal, which was supposed to publish the criticism of the respected researcher, hesitated for a long time to do so. Meanwhile, the university itself set up a commission of inquiry with experts to examine the facts. Apparently, the critic is proved right. This was suggested by a media report. The case is probably not quite closed.
Afterword: There has been more than one case in Germany in recent years in which career-conscious personalities who went into politics wrote dissertations in which material and judgments were used from general information sources - online - that were issued as their own achievements. It is well known today that there are surveillance services that publish this suspicion, if it can be proven, online for every suspicion in this direction. This of course also has institutional consequences.
In such cases, I several times wrote polite letters (the situation concerned not only me, I clearly saw that other authors also ... were used) to "authors". But I received no answers. In my opinion, the "hunchback will be corrected by the grave" (this is not very politically correct Russian proverb, it mean, what it is natural (inherent) action for him, and there is no way to fix it during his life).
Write to the author and express your concern. If s/he turns a deaf ear to address it, report to the editor.
Journals that publish author's papers without conducting any conducting plagiarism/similarity index must be stopped from operation. Why should they encourage academic theft?
QUOTING WITHOUT DOCUMENTING..... these are contradicting in terms. Quoting or quotation entails "xxx" giving some credit already; the writer already acknowledge that what has been written is a quote, and, thus, not his/her own. DOCUMENTING might be referring to formal citation or reference. A quotation may be used, if the subject quote is common, without full citation. For instance, "give me liberty or give death" is a common quote in political science and in the field we know the quote is referenced to Patrick Henry during the American Revolution or the quote 'I shall return" is a common knowledge that it referenced to Gen. McArthur. These types of quote, as long as they are "quotes" do not need full citation. Of course technical statements supporting by finding from an academic paper would need full citation.
PLAGIARISM stands on its own light. The stealing of other people's work should not be tolerated any where.
The first thing to do is to confront the other researcher. If misunderstandings persist, then other prople could be invited to assist in getting the right thing done