Fiction workshop classes are very interesting if you read more than one story by a particular classmate. I tend to have high standards for fiction, but I've also learned to be nice to other writers in class. For starters, negative comments can induce writer's block, tears, and that paralyzing fear to put words to the page. Teachers also do not appreciate cutting remarks; my first creative writing teacher gave me a blunt lecture about two negative critiques I had written, and it was the best advice I had ever been given after making me cry for the rest of class. (I still love this teacher to death, and I highly recommend her if you ever take Creative Writing.)
If you want to be a good writer, you have to be a good reader. Good readers will have a perspective when reading others' comments on their writing as well as seeing what works and doesn't work with fiction. Few ideas are original; so are few techniques. You'd be surprised with how far long sentences can go when reading aloud.
Tam Lin provides the best visual example of your relationship to a fellow writer as a reader: Sarah Beth Durst has a wonderful response to the ballad, but only peruse if you need context.
In a nutshell, Janet the pregnant heroine has to hold onto Tam Lin as he changes from monster to monster; it's the only way to free him from the clutches of a murderous faerie queen.
In real life, you have to be Janet when you read someone else's story for critique. Do you have to be harsh? Sometimes, but remember that at some point your story will be the monster and may be slain quite brutally by that other person. Therefore, you also have to find some redeeming value in the story so as not to crush the writer's spirit. Someone else may crush your spirit, creating an Evil Golden Rule that you want to avoid.
One point to note: always ask to read the next draft. Your friend will appreciate it, probably read your work as it progresses from draft to draft, and improve their story for your pleasure. I have read improved second drafts because I asked for them, and I've learned what writers can do when they're given a chance to improve themselves. You can do the same if someone else gives you the chance.