What exactly did a Fox employee say on the air?
If you haven't got an example, you haven't got anything. Allowing guests to make accusations only makes the guests liable.
If news media could be sued for lies told by guests, there would no longer be any news media.
“Allowing guests to make accusations only makes the guests liable.”
Not if you are aware that the accusations are false, or if you disregard whether the accusations are false or not.