I think you misunderstood the original question

I know that you don't HAVE TO surrender your I-94 when driving from the US to Canada. However, my understanding of the original question is that she wants to have a new I-94 which allows her to stay for another six months instead of filing an extension. An extension is said to hurt your chance of getting visa in the future. Therefore, taking a trip to Canada and getting a new I-94 upon return will help avoid an extension but get another six months. I did exactly the same thing, coming back from Canada (I surrendered my I-94) and from Europe as well. Sometimes, I got six months. But other times, different number of months. Thus, I said that the actual time would depend on the officer.