I would separate them. Don’t think cutting of feathers would help. How old are they? It is unusual for a rooster to go after a chick. Generally they are protective of the little ones. Even mine, though assholes to me and some grown hens have always been very sweet with new immature pullets.
It doesn’t sound good. All you can dobis give supportive care and wait. Put her in a safe environment, so she can’t hurt herself and help her drink and eat. Give extra poultry vitamins if you have them - always good in times of stress. Crossing my fingers for her.