Do what makes you happy. I was 200lbs and am 5'2 when I got banded. BMI 36.6. Many will say that I was to "small" to get this done. Screw them. I have to admit, I went on a eating spree before I had to start my pre-op diet. I was 197 and went up to 200!! I don't have any advice on how to gain the weight, but I wish you luck.
I think its better for our TAX DOLLARS to pay for her surgery now, then having to pay the thousands of dollars that will come LATER from health problems of being overweight!!!!!