As much as I feel for this woman, this will continue and continue if you allow it to happen. She needs to see paying you as part of her important bills, and not "oh she won't mind, she's just doing it for extra money anyways". She can not be late unless she intends on paying the late fee. If you do decide to let this one slide, make it extremely clear that this is a one time only option, and will not be allowed to happen again.
How did you respond to her when she called you?