You don’t have to rewrite their essay or even know the exact mistake -  you just have to say a few words.

1. "I didn’t understand the last part of your second paragraph.
2. "You need a better topic sentence.
3. "You need to write shorter sentences.
4. "Your ideas are really great, but you need more examples in Paragraph Three
5. "Your conclusion was off-topic.
6. "This is not a full sentence: “While you were writing”
7. "Check your punctuation.
8. "I think your essay is well-organized, but…
9. "Your conclusion is too long.
10. "I think simpler words would improve your essay. For example, what does “efficacious” mean?

You can check the official descriptors for IELTS Task 2 Writing (public version) here.

Don't be afraid about criticizing. A good comment that helps someone to fix a mistake or re-organize their essay is much better than just saying "Really great essay" or "This is too cool, dude."  If you want someone to love you unconditionally, get a dog. If you want to learn to write better essays, read other essays, comment on them, and change your writing!