I've been tutoring here at Stony Brook University for three years now, and I'm hardly an expert. I do, however, have a soon-to-be-official professional background in education, and I have enough experience tutoring to see what works in terms of how to talk to students and explain things, and realize that "right" and "wrong" is more often than not very subjective. Saying something is wrong, for example, without questioning why it;s wrong, especially because you were taught that way in high school, is a really bad idea, because a lot of those things that teachers taught you in high school, were.... well, wrong. English is a fluid language, people, and it changes and grows and rules come and go. The English language is based on its being culturally used and those cultures change. That's part of the reason why there's not just one English language; English is the official language of multiple cultures, and the culture that uses a language is adaptive.
These are some the things that go through my mind when I eavesdrop on other people's tutoring sessions. Most of our tutors are at the very least just fine. Some are better than others, of course.... I'm not sure if there are any fantastic tutors around, but some are definitely stronger than others.
Occasionally, though, we just have tutors who are just bad. Not many - but we've had one particular tutor here who is just... not good. She is not patient - which is to say she's not specifically impatient, but there's a better way to tell students that we aren't human grammar checks than, "We don't do that here." I've had students come in, just wanting help with lower level grammar problems (the comma splices and semi-colons, etc.), but I generally approach these students along the lines of, "Let's take a look at what you have, and see what can fix; how's that?" And then I just (hopefully) start teaching them the tips that will help them recognize those problems in the future. I'm not sure how to approach her about this; I feel that she could learn something but I'm not sure how receptive she would be. What bothers me is that she's slightly condescending, possibly without realizing she is, and that is just something I feel is really the wrong stance to have.