Weak skinny chicken

[AMD10]

I am trying to figure out what is wrong with my 2yr old buff Orpington chicken. She’s very weak, stands up but easily falls over when she walks. And so skinny, her breast bone is protruding. I do see her walk over to her food and water without an issue but when I let the others free range, she stays in the run/very close by. She was getting beat up a lot by my Amerucana relentlessly, so I rehomed that chicken thinking it would resolve but it has not. She has not laid an egg in a couple months (obviously). I was looking into that’s going on and I am thinking maybe coccidiosis? But my other 3 chickens are fine, no issues. Any other suggestions?