I agree with doing it the loving, gentle way. When you get to the retreat, you might say to her...I am so excited to get some of my projects done or I am so excited to learn from this teacher. I don't know her techniques at all. I am going to be very focused this weekend and learn a lot. That will let her know that you need to learn too and can't be her go-to person. Also, try to sit on the other side of the room from her. It sounds bad, but she then becomes someone else's problem. Some people don't find it as hard to say I can't help you at this time, you'll have to wait.
I think it would help if you know if she really needs the help or is she just the "center of attention" type of person? If she asks a question and you can point to a specific place in the pattern and tell her to go over that, it explains it much better than you can, maybe that would be a first step to breaking the neediness. Maybe she doesn't even realize that she is doing it. If she needs to rip something, just tell her what to do after she gets finished and return to your own work. The bottom line is to get her to be responsible for herself. Everyone needs to back off and let her fix her mistakes and rip out her own stitches. You can do this without being mean. As a southern lady, it is hard for me to be mean to anyone and too easy to be helpful, which actually hurts the person.