I didn't read all the comments so someone may have already said this....
I would do it because she will learn to eat healthy and learn portion control now while she is young. Things that we learn while we are young, especially habits, stick with us our entire lives. If she learns portion control now it will stick with her forever.
I would suggest that she prove to you that she is serious by learning to eat healthy now. She can be losing while you go through the surgery, then once she SEES what you have to endure and helps you while you to through this very difficult surgery then if she still wants to proceed and she has been successful in learning how to eat healthy then I would let her proceed.