Bring him to a seminar or something so he can understand what the surgery is instead of what he assumes it is. Other than that I would suggest losing a ton of weight now by dumping his ass.
I usually just respond with something that makes them want to put their foot in their mouth like "Well, it doesn't look like I don't need it but my liver disease says otherwise." Then maybe she will think twice about those kinds of comments.
Let her be aware that it is so difficult to lose weight when you're older that's why some people need a tool like the sleeve. Also I like the suggestion earlier of taking her along for seminars and doctor's visits so she can understand what's going on.