Don't take a grade hit for a "friend" who could not care less. Email your teacher the night before the assignment is due, your concerns about your grade due to your partner's lack of effort in helping you complete the assignment (Assuming your friend just doesn't do the work.) I suggest writing an outline for the sections you were responsible for, and the sections your friend was supposed to write. Since you are two different writers, and you clearly care about your grade, it should be obvious just by reading the essay that there are sections more complete and thought out than other sections.

Your friend needs a life lesson in not procrastinating on important assignments, especially when another person is relying on her to do the damn work in a timely fashion. Since it is Sunday, you could gently remind her/ask about her progress on the assignment. Tell her exactly which sections you expect her to work on and the ones you will do on Monday.

Or go nuclear. Tell your partner she is on her own for the assignment and that you are completing your own assignment by yourself. Either she gets pissed and starts freaking out on you and ends the friendship, or she gets pissed, gets stressed, and comes crawling back apologizing and saying she will do the work, please help her.

Whatever you do, don't be a fuckin doormat. People outgrow their friends sometimes, especially around your age. And your "best friend" doesn't sound like a very good friend or student.