Writing is never an isolated activity and it is common practice to ask others for feedback on your work - or for others to ask you to take a look at their writing! Being helpful but also tactful is always the best route. No one wants get their paper back with two typos marked and a "looks good" note, and on the flip side, no one wants to get their paper back covered in so many comments and suggestions that it is impossible to find the original text.
These guidelines might be helpful when giving feedback, until you develop a feel for it:
Reading others' writing will improve your own writing and help you spot your own strengths and weaknesses.
One of the most stressful parts of writing is receiving feedback. Writers often (mistakenly!) equate their success or failure on a paper or project to their success or failure as a human being. Writing is a skill like any other, and with practice it can improve. It is entirely unrelated to your worth as a person.
Three common reactions to feedback are anxiety, defensiveness, or frustration.
Remember, the comments are about the paper, not about you.
Writing is a process that takes time, practice, and patience - that includes patience with yourself!