Hattie, I just love your flock. They are so gorgeous! I think even if you do plant them some kale, you'll have to keep them out of there for a while for it to really take off and grow before they start nibbling.
JesusFreak, the others gave you the very best advice. That's the very best way to do it. However, I understand that sometimes that just isn't possible. So, I would check the hen over very carefully for lice or mites or any other disease. Then, if you HAVE to put her in with the rest right from the get go, stick her in the henhouse at night when everyone is sleeping.