Maybe this topic has been broached before, and I am positive there will be many different opinions, but how far do you go [how far do you think a reader SHOULD go] when evaluating an essay?
There are many grammatical errors. Tell the kid to find them & fix or specify what they are?
The essay is disorganized. Say so or suggest how to better order the concepts?
Follow me? I can write a great essay at this point, but that’s not what I should be doing (IMO). Where do you think the dividing line is between helping and rewriting?
And, is there a difference for students whose first language is not English? I think it’s totally fair to point out an incorrect or inapt idiomatic phrase. What about a slightly off grammatical issue? Is it better for the applicant to learn (a teacher could certainly do this) and fix the off-tune sentence, or is it better for the college to see that the applicant may not have the language down pat but that the thoughts being expressed are sound?
Discuss.
An editor should go as far as they need to in order to help create the best essay possible.
A great essay should be free of grammar and style errors, no exceptions. You should point these out unless the paragraph needs a serious re-write.
A great essay must be well organized and well paced as well. You should point this out, and suggest for ways to order thoughts better.
You should never re-write any portion of the essay, unless it’s to suggest a better wording for a particular sentence.
When I revise a paper, I don’t care what the writer’s first language is, since readers in admissions frequently don’t take that into consideration. Obviously they should, but they don’t.
I used to put an X next to any line with a spelling or grammar error for my kids when reviewing their essays. But they had to find the error.
I would give feedback on:
- Does the essay fulfill the prompt and tell a compelling story/get the point across?
- Transition issues
- Awkward phrasing or incorrect word usage
- Quality of opening and closing
I tried not to rewrite, as it wad their essay and their voice. But some of them went through a LOT of drafts. Some of those Xs drove them crazy. 
But as they are my kids, I was fine with that.
D had an excellent professor at Dartmouth (actually, almost all of them were excellent) who would make annotations in the margins like D.P. [dangling participle] and such. I learned a lot just from looking at a couple of her first papers.