That is very nice of you to take care of Libby.
Most of the Boston Terriers I meet are a bit pudgy, but I recently learned that they are meant to be thin. They should have legs like a greyhound. However, if her ribs are visibly protruding then it does sound like she is underweight.
If she is eating a good amount of food, the only thing I can think of is to add more calorie dense food, including some fresh foods. But then I would be worried if she has a tolerance for heavier foods.
How long has she been eating TOW? Maybe with a little more time the food will help her put some weight back on.