Editing in an Academic Journal TLDR: Can a researcher include an entire paragraph from another article for which they were a co-author on a new, single-author article?
Copy-editor for an academic journal here with an issue I've never come across before. We work with Chicago-style footnotes.
I have a solo-authored article by someone (let's call them Alex) that includes an entire paragraph from another article in which Alex was a co-author. I only found out that the entire paragraph was lifted from another article through my own background research. Giving Alex the benefit of the doubt, I asked how they would like to address this paragraph: Would they like to rewrite it? Cite the paragraph in a footnote? They would like to cite it in a footnote; however, I am now unsure of whether this is an appropriate fix given that it's an entire paragraph from another article in which Alex was listed as one of many authors, and for which Alex was not mentioned as one of the writers. I'm not sure what to do given that an entire paragraph seems like it's enough to warrant including all the other authors from the other article.
Here are the questions I'm wondering about that will help me determine what path to take with this article:

Do I go ahead and cite the entire paragraph as a quotation? If so, do I need to tell Alex that they need permission from all other authors in the previous article?
Do I tell Alex that they need to include all the other authors as co-authors on this paper if they want to include this paragraph?
Should I just change my mind and say that, after reviewing best practices or consulting with others, Alex needs to rewrite the paragraph?
They need to give a citation, but, if it is permitted to quote the entire paragraph, then they don't need permission from the other authors.
Whether it is permitted or not is a question of copyright and fair-use exemptions. If they still hold copyright jointly, then each of them needs to give permission. If your journal holds it then I see little problem as the journal can grant a license. Otherwise permission should be sought from the copyright holder for a long quotation.
Even if the old paragraph is paraphrased rather than quoted, a citation is required. Otherwise it is self-plagiarism. In that case, the copyright issue would disappear as there is no &quot;copying&quot;.
But I don't see any reason that the other authors on the old paper need to become co-authors on this one. Certainly an individual can extend old joint work as a single author.