I felt awful when I ended my study. (especially since just the study before that, I had said I wanted to become a witness and get baptized) So I broke her heart, and I knew it, but I also knew that I couldn't continue for HER sake. It had to be for God, and for me, and for truth. I told her that, and of course she agreed.
In my case, I just said I needed to stop our studies. That I needed time to study on my own, without ANY talks or interference or teaching from our study sessions. I guess I was assertive, because she packed up and left right then. She came by a couple of times with magazines and talked a bit just inside the door, but she respected my no study time, and we left it at that.
We e-mail sometimes, though we tend not to speak about religion or faith much. A little... but neither of us challenging. I spent more quality time with her in two years, than with anyone else in my life (other than my children). I feel we both grew to care for one another. She even said she doesn't care what 'religion' I am.
I haven't seen her, and since I lost my e-mail and all my contacts, haven't talked with her in a few months either.
I feel more like I'm letting her down now. I haven't invested nearly as much time in her - witnessing to Christ as I understand Him- as she did trying to 'save' me.
I think you will have to say no more study, and no more magazines, and just be done with it. You don't have to end your friendship with her. Unless she chooses to do that, herself.