Hm, that's interesting that your farrier said her hooves were nice a hard, yet she was sore. Did he have an explanation? This makes me think it could be something else besides just tender hooves. Do you know if she's being trimmed correctly?
Here is a website about the natural horse hoof trim, this is how I have my horses trimmed: Barefoot for Soundness
I recommend looking at the 'horseshoes' link. Just some food for thought.