If the sentence has a clear typo, fix the typo. If it has a grammar error, fix the grammar. If punctuation makes the sentence hard to read, add punctuation. If the wording is awkward but understandable, ask whether the writer actually wants a style change.
If the sentence is unclear, then a limited rewrite may be justified. But even then, keep the original claim, audience, and tone. The difference between careful correction and paraphrasing is the amount of control you keep over the original wording.
This is why correction-first editing works well for people who already know what they want to say.