Well, I was "scandalised" to find information in favour of your proofreader's opinion! On the ProWritingAid site I found this explanation:
Sometimes "for" can be used as a subordinating conjunction in place of
"because". If it starts a dependent clause after the main clause, it
shouldn't be preceded by a comma:
She bought more gloves for she was always losing them.
However, if the
independent clause that comes before "for" contains a negative verb,
you need a comma:
He didn't take his umbrella, for the rain had stopped that afternoon.
On another site I found a more determined "pro comma" opinion:
When “for” is used as a conjunction between two clauses (“for” is the
F in FANBOYS) then you need a comma before it. When it’s used as a
preposition (e.g., some flowers for my mom) you probably don’t need a
comma before it, unless it’s in a list or some other structure that
I also found a rule on the PurdueWritingLab about FOR linking two independent clauses:
Use a comma before a conjunction (and, but, for, nor, or, so, and yet)
to join two independent clauses.
Use commas to separate independent clauses when they are joined by any
of these seven coordinating conjunctions: and, but, for, or, nor, so,
So I presume it is finally a matter of deciding if FOR links an independent clause to a dependent one, or two independent clauses.
Here is the guideline I would personally follow:
A comma always comes before the fanboys when they introduce an
independent clause or a complete thought. However, if the fanboys in a
sentence do not connect two complete ideas, then we do not use a comma
in front of them.
Note: FANBOYS is made up of the initials of the seven coordinating conjunctions: For, And, Nor, But, Or, Yet, So.