"Why don't you try it and we'll see what happens?"
or
"How fat are we talking here?"

It depends how "fat" she is already. If she eats well and exercises regularly, then her getting fat would be due to a lack of discipline. We'd have to talk, as our goals might be distant from each other's.

I don't think her percentage of body fat has anything to do with whether or not I'd stay with her. However, repeated occurences of her asking this question would probably cause me to ignore her... permanently.