You don’t necessarily have to act as a writing center, but certainly mentioning mistakes you noticed would be fulfilling a crucial education aspect of a reference librarian’s position at an academic school. You might even be able to help them find other resources such as writing center, English as a Second Language support groups, or material that could help the student to write better. seems cruel if the student isn’t aware of the mistakes to ignore them only for the instructor to mark down later. Why help with researching but not writing?

This is a guess, but one possibility that comes to my mind is that it is possible that this is someone for whom English is not the native language. In academia, we run into students from other nations who have good thought content but who are still learning how to express those thoughts in proper English. When helping students at the reference desk, I often see papers that have good ideas but bad grammar. It’s somewhat of a moral dilemma whether librarians should help the student “fix” the paper or not. And I know that if I were blogging in Korean, someone would be saying the same thing about my posts as you are about this one.

On the other hand, if it is a native English speaker, then you have to figure out if the sloppiness is intentional or not. In aeither case, I would have to agree that it detracts from the message that the person is trying to send.