Wow, Perna has hit it on the nose in my opinion. It really isn't about what we're afraid of "happening" in regard to weight, it's about what we SEE before us. As a person who loss over 150 lbs over seven years ago and now is a a "normal" weight", I used to have a hard time dealing with individuals who were overweight after losing my weight. It wasn't that I couldn't be sympathetic and empathetic about being that weight (god, I could surely understand how hard and heartwrenching that place was to be), it was about whether or not I could relate to them as being in a TERRIBLE place mentally in regard to their own eating and numbing out.
I know that I am totally lucky in that I have a therapist who works with eating disordered individuals . .. along with other issues

, and I know that, even though she could NEVER have known what it was like to be in my weight range, she understands eating disorders on a very close and imtimate level. It gives me comfort and reassurance.
I think it's great that you've already decided to discuss this issue with your therapist, geez. Sometimes, those of us who have lived a life of being overweight believe that NO ONE can understand our pain . .. and if another person is overweight they can't understand because they CAN'T GET A GRIP ON THEIR OWN ISSUES. One thing I've learned over time is that there truly are overweight people who are comfortable and happy with their bodies. Just because I have always hated my overweight body doesn't mean that others feel the same way. Self esteem and self confidence comes from many different places within us. I hope you don't discount her for her weight. She might surprise you in a wonderful and wonderous way.