I would give her a/d once a week, dry food helps to put weight on more easily than wet, and you could try cat milk and extra portions. IT is much better for them to gain weight slowly though, and you need to make sure that when she reaches an ideal weight, you can taper off the extra things to avoid her becoming overweight. Have they done any blood tests, to make sure it isn't something like her kidneys or thyroid that are causing the issues?