Yogurt has acidophilus, which is a probiotic. This means the bacteria in the yogurt will help re-establish good bacteria in the digestive. Yogurt would be a good thing to feed after Talia is done with oral antibiotics, which I'm assuming she is getting for the URI.
Digestive enzymes are molecules which will 'dissolve' certain foods. For example, papayese is an enzyme from papayas which will help break down protein. My recommendation is to go to a local healthfood store or online and buy mixed digestive enzymes. A mix will contain different enzymes to break down protein, carbohydrate, and oils. (One enzyme, one type of food.)
Unfortunately, I can imagine someone feeding her only treats. (I'm getting too cynical these days.) You might want to ask this lady what kind of treats. The reason is if Talia ever stops eating, you can crush some of her favorite treats on the food as a bribe.
Given that she was on such a poor diet, you might want to supplement her food with a cat vitamin. She has to be very malnourished and extra vitamins for a few weeks won't hurt. (I suppose I couldn't talk you into raw food?
And yes, I can imagine a cat living under a sofa. I don't know for sure, but I would bet my Isabel lived under a bed for 1-2 years and only came out at night. This probably had to do with a toddler in the house. I just don't understand why people get these cats in the first place.