The line from Helen Grey, which implies that the author thinks Helen's behavior is going to hurt herself is this: DON'T CUT YOUR FINGER WITH THE EDGE OF YOUR KEEN WIT.
Cutting one's finger implies hurting oneself or doing harm to oneself. When a man is said to have a keen wit, that means that, that man has a very sharp intelligence. So in the line given above, what the line is saying essentially is that 'Helen should not hurt herself with her sharp intelligence'.