We don't just point out errors or tell writers how to fix them. We ask if they can spot an error in a sentence. If they can, then we know that they just need to be more careful to proofread for themselves. But if they don't, then we ask them what rule they used, let's say, to put a comma where they did. Often writers have memorized rules that are incorrect, and once we look up the correct rule in a handbook, and we check to make sure the writer can find those errors in other places, the problem is cleared up.
Sometimes the writer has serious problems with error. Then we can make a standing appointment to work on correctness. We'll begin by working on the most serious errors (the kind that interfere with comprehension) and make our way to the fine points of gerunds and dangling modifiers.
International students sometimes need help with English idioms and with articles (like "a" and "the"), because these elements are not found in their first or even second languages. Idioms don't always seem natural, and the rules for articles are so complex that even the best flow charts don't predict all the exceptions. So we help writers with these things. Articles are often the last things these writers master, and it takes many years of careful listening to English, reading, watching lots of TV, but mostly talking in English. So we encourage international writers to form friendships with native English speakers.