I know how you feel. One of my friends that I really expected to support me, acted the same way. My response was: If you are not going to support me, then we will permanently remove this topic from the table, and ill keep this part of my life to myself". Simply -- and we never talked about it again.
I am still Pre-Op, so I can't offer MUCH advice about the surgery, but I did read a posting on here, written by a Personal Trainer that said to start working out before hand (not cardio, but strength training) to build muscle now, which will in turn start tightening your skin and of course muscle burns fat He even listed a website with tutorial strength-training videos (www.bodybuilding.com/exercises/). This is what I will start doing NOW and continue to do after surgery. There are exercises that will also work on your boobs
Lastly, every single person that I have seen lose weight (from exercise, surgery, diet, etc.) has looked YOUNGER afterward, so please do not let her discourage you in ANY way. You know what you need to do for your health. Stand by your decision, and cease to discuss it with her anymore. I know its hard becasue she is your best friend -- but it will not help you, and right now you need support.