So this is a hen that was new to the flock that you integrated? I have a similar problem with a hen I adopted from a neglectful owner a couple years ago. My rooster never would accept her. He doesn't all out attack her but if she goes in the coop with the flock he will give her a hard peck. She now lives with a buddy in my goat barn. It sounds like you are either going to have to rehome the hen or the rooster if he is attacking her that badly.