Thanks for his age, that helps. What I think is happening is that the immature cockerel has matured to the point that he is trying to take over as flock master. That hen was the dominant hen and flock master. She did not want him to take over so they fought. He won but she hasn't totally accepted him as flock master so he is intimidating her until she accepts him. Intimidating means he is probably attacking her out of the blue, pecking her or worse, and running her away from the rest of the flock. She is scared and rightly so.
Different things could happen. Since violence is involved she could be hurt or killed. They may work things out. I had this happen once. For two days it was pretty brutal, then they sorted it out and became best buddies. She was still the dominant hen but he was flock master. No blood was drawn so I let them try to get through it.
It is pretty normal for an immature cockerel in a flock of all hens or pullets to take over as flock master when he matures enough. I had one that did that at 5 months, I had one that took 11 months. A lot of mine manage around 7 months but it varies by the personality of the cockerel and the personality of the hen. It sounds like you have a stubborn hen and a somewhat early maturing cockerel.
Most of the time when I see this transition of flock leadership it is pretty peaceful. A lot of the time I don't see it happening until I notice he is now the flock master. But every now and then you get two that clash.
Eventually this will work itself out. She may rejoin the flock or she might be injured or killed. I don't know what will happen in your specific case.
What do I do? I really didn't want a roo but he is nice and beautiful.
The only reason you need a rooster is if you want fertile eggs. If you don't want fertile eggs the obvious answer is to get rid of him. Now.
If you decide to keep him you can let them work through this and see what happens. Keep an eye on it and be ready to intervene if necessary.
Or you can lock him up where they cannot get to each other but can see each other. Leave her with the flock, lock him up. When he gets more mature she may accept his dominance without a big fight. In another month try again and see what happens. If it doesn't work, do it again for another month.
Or you can lock her up for a while, at least a week. Again, I think it would be good if they can see each other. See if there is any interaction between the two through the wire. Things may calm down between them enough for it to work.
If you decide to try to keep him I don't know what will happen if you try any of these. They might work, they might not. Base what you do on what you see because anything can happen.
Good luck.