Try a golf ball. Mine has broken some real eggs and ended up a mess. Check her nest for real eggs. My other hens will all go lay eggs in the broody hens box when she is in there and then she sits on them all. I just take them each evening. She gets grumpy so I just leave the fake one.
You can try breaking her. I don't think it's harmful I have just never been successful at it and she always seemed very upset. But I know it works well for some people.
