If blood has been drawn, it's really important to segregate her somehow, because the other hen or hens will carry on pecking at it and make it worse. In the worst case, they may even kill her. Also, it may establish the other hens in a feather-pecking habit which otherwise they wouldn't have considered doing. Can you get some sort of mesh or chicken netting and fix this across the run and put her one side, with food and water, so she can see the others and they don't forget about her? this is important as if she is kept out of sight there will be reintegration problems and probably more pecking when she has recovered and can be returned. All of us have to separate a hen sometimes, for various reasons, so it's good to make up a 'kit' which you can use when necessary. Alternatively, if you think just one hen is doing the pecking, remove the bully instead so the bullied hen doesn't have to have the stress of separation. It's possible that removing a higher=ranking bird will result in her losing a notch ot twp of her place in the hierarchy when she is returned. This demotion may help solve the problem, besides giving the bullied hen a bit of respite to heal up whilst the bully is not there. But if you have several hens and they are all having a go, then the only thing to do is to remove the victim.
The only other thing you can do is to try feather-pecking spray on the afflicted hen, though this is horrid stuff and doesn't always work. Also, the hen who has been sprayed can't preen her feathers properly and so she suffers for the sins of the other aggressors. If you can separate her instead this would be a much better way to deal with it.