Don't assume that weight will be a sensitive subject because she is fat. Some women are happy in our own skin (amazingly, given the pressure for us to change ourselves to fit). If you do offend her, she will either tell you and you can discuss it or she will take it to her own support sources. Either way, it's your therapy so speak about your issues, don't try and manage her feelings for her. This could be a chance for you to develop social skills and see how the boundaries of your self interact with others.