First of all, I think it shows incredible inner strength for you to recognize that your friend's response was controlling and unhealthy, and even more self-control for you not to respond to her right away.
I would give her a call and make it clear that you are not asking for advice as to whether or not you should get bariatric surgery. It has been recommended by your team of physicians and you will be following their medical advice. That is not what the call is about. I would state explicitly that you plan to surround yourself only with encouraging and supportive voices over the next year or so, as you go through this transition into a healthier you. Any person who is negative toward you will have to wait a few years before they can re-enter your life. Your contact with her is to see if she is interested in being one of your cheerleaders during this difficult process, because she has been such a strength to you in the past and she is someone you hold dear to your heart. While it seems counter-intuitive, it may help in the long run to allow your friend to feel heard. She has heard some negative stories about bariatric surgery, and it would help both of you to let her get it off her chest. I would ask questions about dates, what kind of surgery it was, how much weight was lost and regained. Ask if you may be able to speak with these people to find out what complications they had. I am guessing that once she airs her concerns, she will be better able to listen to your point of view. I hope that she would then ask you why you are considering bariatric surgery. I would be concerned if she did not. If she continues to behave in a controlling way toward you, I would thank her for her concern and perhaps even mention that you are disappointed by her response, and then wrap up the conversation. I hope you can find wonderful cheerleaders to be there with you through the process, in addition to the friends you will make here. There may be a support group in your area where you can meet people.