Hey!
Our former rooster, a Welsumer, has attacked a Buff Orp pullet we have some three times. He would have killed her. He really, really seemed to hate her.
She had been ill and got over it, but was left very submissive. The first time I realised he'd he attacked her I found her head a bloody mass and once she'd recovered she was blind in one eye. I tried to separate her from the flock with a friend or two- hens who couldn't be with the rooster (one with a damaged leg, one another Buff the rooster didn't like. The rooster definitely did not like lighter-coloured birds.) She wasn't happy so I put her back in with the flock in a dog crate at night and then roaming with them during the day. I kept a watch on things for two months. All seemed well.
Then one night the Buff didn't come home at sundown and I found her lying out where he had left her, her head a bloody mass and now blind in the other eye. So I separated her with the other two birds. She made an amazing recovery (we were going to euthanize her but our young son was beside himself. He loved Winky and she was very friendly and sweet.)
And then... one day while the flock were freeranging that flaming rooster flew over the six foot fencing into the former goat pen where we now kept her and her friends, and attacked her in the same way again. Again she recovered and ever since I've kept her and the other three in a completely sealed over run. And when he was out ranging that rooster would still walk the perimeter of their fence and try and attack Winky through the chainlink. Where on earth he found that venom (he was otherwise a lovely, good rooster) I have no idea. I'm rehoming them with some good 4H friends this weekend hopefully.
Long and short, I think that roosters decide they don't like a hen sometimes: for her looks, for her submissiveness, for her in some way inferiority or unacceptability to the flock. I think you shouldn't put your hen back. I would bet very sad but sure money that it would happen again. Plus your hen is now primed for terror and will attract more aggressiveness by her flinching. I'd rehome her or house her separately or range the flock without the rooster and only then let her out too. However on that note, I've found that my three 'Winky hens'- the three who can't be with the rooster- don't feel comfortable running around outside with the larger flock anyway. They can all still see each other, but they don't socialise the way they used to. It's made ranging them all very complicated, and the job for animal sitters when we're away really fiddly. Hence I'm rehoming them. At least I won't have to worry about poor, brave, sweet Winky being beaten half to death on a whim anymore.
