From your rooster's perspective, an entirely new, alone hen has just entered in and is expecting to eat and sleep among HIS women. He likely isnt registering her as an old friend. If he's not drawing blood or pulling out feathers or harming her, he's just trying to put her at the bottom of the pecking order. He might have reacted similarly if she had brought chicks with her, he might not have.
Are the other hens accepting of her? If it's just the roo, you could separate him for a week or so and let the hens regroup without him. If it's also the chickens you might want to look for threads on here about how to stop chickens from abusing other chickens.