I agree with others, you did the right thing by telling her. I know if I was in that situation and came home to no power I would have been so upset that my friend hadn't even told me! I would be embarrassed for sure but atleast would have had prior knowledge as to what I was coming home to.
As far as it getting turned off, here in OKC it will be shut off if not paid in the 7 days following the due date. Doesn't matter how little or how much is owed either. I think you are a very good friend and I commend you telling her, no matter how awkward it was to do that.