If she already has a bald back you need to check her flanks (look under her wings) to make sure that your rooster hasn't injured her.
She probably ought to be removed from the flock until feathers grow back anyway;or you could buy, or make, a hen "saddle" if there are no other injuries.
Eventually, if you leave her in with the offending roo and do nothing about the bald back she will probably get hurt. I learned this the hard way and my hen's sides were torn open. Those missing feathers protect flesh from sharp spurs. If you are in a cold climate then your hen is also losing precious heat (and moisture, as well, I'm told) from an uncovered bare back.
Even if she's not visibly injured, & in no danger of losing too much body heat, then I would seperate her anyway for a few days. If she's seperated it is easier to keep track of food consumption & H2O intake & what her poo looks like (normal, runny, bloody etc) which will possibly indicate different illnesses. Add some soluable vitamins to her water (feed store should have these), sometimes this stimulates appetite. As for food, try a little cooked egg, some oatmeal & a little spinich along with regular feed. Mine always go for few kernals of fresh corn (canned or cut from the cob-but just a few kernals) just enough to stimulate eating and get a little carb boost.
If you seperate her & her behaivior doesn't change, the poo looks really abnormal or she continues to refuse to eat then you need more help & may want to consult a vet.
Good luck.
