Just don't try to stuff her with food right away. Start with a proper sized meal, 15% of her weight should be okay, someone can correct me if I'm mistaken. Both of my BPs were severely under weight when I got them. One was 2yrs old, 30" and barely over 200g, she just broke 400g finally. The other is approx. 4yrs old, a little over 30" and 360g. Both of their spines were sticking out so far you could feel every individual vertebrae and rib. Start off with smaller meals, I have them on 5 day feeding schedules, seems to be working well. As they gain weight you can up the meal size. My 2yr old started with medium f/t mice and is up to f/t rat weanlings.

Patience!! You'll be surprised how quickly you notice them filling out, though.