I know this probably sounds rude, but why in god's name did you give them back to her? I'd of told her to go get animal control, have THEM decide if the horses were "starved" and work through them; otherwise, she could go f**k herself.
I've never been in this situation before with horses, but I've been there with other animals. I bought a lot of animals (15 goats, 1 ram, 1 mule) from someone two years back for $115, and I was told that was all I'd have to pay, as she was desperate to get rid of them.
...Then she came back and demanded more money, claimed I'd promised to pay ten payments of $115... even when half the animals did NOT live up to her claims (one goat she claimed was 4 was actually closer to 8; ones she claimed were tame were aggressive, to the point of one attacking me; the ram had serious medical problems; the mule, although sweet, was sold to me as a donkey and DEFIENTALLY a mule).
I ended up keeping the animals. She couldn't prove her case, even after she changed tactics to the animals being mis-cared for (the ram's health problems being my fault, the one goat being underweight, etc.), it didn't hold up.
I ended up losing them a year later, though, in a HORRIBLE 1-month-long boarding experience, but that's a differant story.