This is a tall order: teaching an 11 year old who has bad habits.
To accomplish this you need two skills:
1. Know what the good and bad habits are yourself, and know how to show/impart them (that is the easy part).
2. Have the technology to convince the 11 year old to change her way – which by now is very comfortable – of playing to a different way of playing which will feel utterly unnatural and positively wrong. By the way a lot of ”professional” experienced teachers are at a loss here as well.
If the 11 year old happens to be exceptionally mature, is aware the she has bad habits is determined to replace them with goodd habits, and regards you as an authority having nothing but admiration for your playing and an irrepressible desire to emulate you, then of course all she needs is a little guidance from you and she will excel.
Unfortunately such ideal students are far and in between. Now comes the interesting point: It is this sort of ideal student who will actually be at most risk if they get a teacher who does not know much, or a “bad” teacher, because they will actually follow instructions to the letter and work industriously on the teacher’s directives. So if she is this sort of student then definitely suggest to mother to look for a really good teacher (especially if you think you can “ruin” her).
However – and I am guessing wildly here – you probably have in your hands a little brat who will not follow any of your instructions, who is quite pleased with her progress so far (the necessary condition for teaching only appears when the student is dissatisfied with his/her own progress), and who most likely will display the very common attitude: “I am going to play in any way I please. What does this guy know anyway?”
If this is the case, rest assured, you will not be doing much damage (and possibly not much good either).
And from your post I gather that you will not be getting any (or much) payment.

So before you take on this assignment, consider this: What are you going to get out of this deal? Because unless this is clear to you, and you are satisfied with what you will be getting, very soon teaching this sort of student (assuming that she is this sort of student – I may be wrong) will be a major energy draining. You will start resenting the assignment and hating the student. Not a good situation.
Here is my suggestion if this is the case. Instead of focussing on the student, focus on your own teaching skills. Accept this student as a way to gain experience in teaching this sort of student. Approach the assignment in a spirit of curiosity: “What can I do to replace this student’s bad habits with good ones?” Consider that you will have succeeded, not if the student has become perfect, but if your own ability to teach has increased. In this way, you will be getting a lot of experience and knowledge out of this deal Does that make sense?
Teaching impossible students (assuming that she is an impossible student) can be quite exhilarating and empowering as long as you focus on your results and your improvement rather than the student’s. Funny enough, by doing this you will probably be giving her superlative teaching as well.

I hope this helps.
Best wishes,
Bernhard