I think this all comes down to the one question:
Does the writing work?
Fiction writers often attempt to escape critique (and criticism) by saying, "well, who can say if it is right or wrong, it is Art."
Take It To An Extreme
Okay, let's take that to an extreme. Imagine if I created an a piece of writing that was so original that it used a new language. Here's my story:
Igto fraxim gelsim dee, hopta angko mazzin frim. Zilla tark il niteo
arv vis a erfto.
Stop Being Ridiculous
Now, you could say, "Stop being ridiculous." I could respond, "I'm an Artist and I'm ahead of my time. Just wait, 50 years from now, people will be reading my stories and begging for more." Who can say? And the writer escapes critique.
Now We Can Talk About Writing That Works
Well, I would say that fiction writers who say, "Well, who can say whether it is good or not," are in the same boat as the previous example.
That means we need something measurable then to use as a guide to give a writer an idea of whether or not her writing is great or good or terrible.
Suggested Categories of Critique
Here are a few suggested categories which can help you evaluate and provide some actionable changes for the writer.
Can you tell what the writer is attempting to communicate? As simple as that. If you have to re-read the piece for meaning or simple understanding explain to the author where you got stuck and why. Was the entire thing cloudy? Let the author know. Writers must be clear.
Did the author take a lot of tangents? If the author is writing about the mating habits of the swallow tail butterfly in her novel about a bank robbery she is not writing a novel about a bank robbery.
Stimulus / Response Writing
Authors are often confused about how events happen.
You may read something like:
The boy was injured by the speeding car.
Sam was humming a song as he stepped into the street. He looked up too
late to see the red Chevy Bronco bearing down on him. He screamed as
the vehicle slammed into him.
One thing should happen which stimulates another thing. Does the author consistently miss this? Let him know. You can learn much more about this in Jack Bickham's great book, Scene & Structure (at Amazon).
Authors who jump around confuse readers.
When an author writes the next thing that the reader is thinking because she's led the reader to think the next thought, then she is writing great.
The sounds that the words make in readers' heads actually matter. That's why some writers can write anything and you want to read it. Does the author take this into account or are sentences difficult to read.
What is the overall tone? Did you enjoy reading it? Let the author know.
There are many more actionable types of criteria you can use, but if you used these you would provide the author with many great actionable items she could use to make her stories better.
You can learn much more from Gary Provost's great book, Make Your Words Work.