Don't depend on anyone. Go with her when she wants, but you need to not worry about what she does. I've had partners in the past and they mostly drop off the plan. Set your own goal separate from her. You need to get use to going alone.
Dont give up on her, tell her how hard it is for you to stay focused when she does not seem motivated - then if she isnt willing to be supportive and step up a notch distance your journey from hers, not your friendship but you have to think about your goals and our journey - sounds like she may not be the best of weight loss buddies.
It is easy to get dragged off track by someone else.... you have to be walking in teh same direction to be jointly sucessful.