I'm spending part of my afternoon doing the admin. on allegations of
student academic misconduct. Hence, this is a topic I feel strongly
about.

Guido says the objectionable material consists of "several sentences".
As readers will see, I tend to write short sentences.

My paragraphs are also often short.

So the questions are "how many is 'several'?" and "how long are they?"
In short, what percentage of the overall work is unoriginal, and does
the unoriginal material come in a small number of substantial blocks or
smaller, more scattered, phrases?

In my institution staff are invited, in the case of student work, to
judge whether objectionable work is a matter of "poor scholarly
practice" (i.e. inadequate citation and referencing), or plagiarism, in
the sense of a deliberate attempt to pass off another's work as one's
own.

The procedures and remedies naturally differ, depending on the outcome
our professional judgement leads us to.

Personally I think the journal editor should take a more active interest
in the matter, starting with an attempt to ascertain the whether the
problem is plagiarism or only poor scholarship.

Since it seems that they do not want to in the first instance (and
assuming that in the light of my points above, you consider that it is
indeed plagiarism), what exactly do they think are the possible outcomes
of your approaching the author, and what would their response be to
these?

Presumably they don't envisage the author blithely confessing to
plagiarism so that you can then report them to the journal editor, or to
their employer.

I'd apologise for not offering a simple answer, but for the fact that I
think there's no such thing.

Julian Wells