Copperhead snake bites are known to have necrotic effect causing severe damage to the blood vessels etc., so the skin peeling off would be normal :
http://copperhead-snake.com/
She might have got bitten by some venomous animal (snake, scorpions, spider) or she might have ingested something poisonous like ergot or similar.
Her legs are proceedingly necrotic and I do not know any means to stop this, unless you know exactly the cause to get some appropriate antidot in...