When I had something like this happen it was a fairly weak-spirited cockerel that finally matured enough to want to take over the flock at 11 months of age. Most f my cockerels do that at 7 months and it is fairly peaceful. The flock leader was the dominant hen and she did not want him to take over. He could mate the other hens, they would squat for him, but not the head hen. If she saw him mating another hen she would knock him off. She was the boss and wanted to stay that way.
When he finally stood up to her she fought and was defeated. For two days he kept her as an outcast, would not let her join the flock. If she tried, he'd chase her and grab her by the head. She was not seriously injured so I let them work it out. After two days of warfare she finally accepted his dominance and he let her back in the flock. She was still the dominant hen but he was flock master. They became best buddies, often hanging out together.
The fighting was vicious, either one could have been seriously hurt or killed, especially her. I did watch for that and was ready to separate them if necessary.
I don't know what would work for you. Letting them fight and work it out certainly has its risks. Separating him for a while might work. In your situation I'd probably separate the hen for few days to see how he gets along with the others. Separating her often will knock her down in the pecking order so she is no longer the head hen and is less likely to fight. None of this is without risks. It is possible you will need to choose between the two.