It's definitely a mixed breed. The owner actually thought it was an Ameraucana when he bought it which probably means whoever sold it to him sold it as an Ameraucana. Since hatcheries commonly and incorrectly market their Easter Eggers as Ameraucanas, I think this one was likely a hatchery bred Easter Egger. With those whitish legs, she may even have some Marans in her and be an Olive Egger. As you said, her condition is so poor, it's hard to tell what her exact feather coloring would be.