You're not the writer; you're a reader. One of many. The paper is not yours; it's the writer's. She writes. You read. She is in charge of what she does to her writing. That doesn't mean you can't make suggestions. It doesn't mean you can't offer a few sample rewrites here and there, as models. But make it clear they're samples, models. Not rewrites. Not edits. Not corrections
When one attempts to annotate someones work either they get too scared to make any suggestions or they are too harsh in their criticism. The reading above creates and paints a nice grey area for readers to follow when they annotate.