I looked up the definition of "slander" on the Legal Dictionary, and it said the following:

"To recover in a libel or slander suit, the plaintiff must show evidence of four elements: that the defendant conveyed a defamatory message; that the material was published, meaning that it was conveyed to someone other than the plaintiff; that the plaintiff could be identified as the person referred to in the defamatory material; and that the plaintiff suffered some injury to his or her reputation as a result of the communication."

I don't know how difficult it is to prove that last part, damage to a reputation. She might have a case, but I don't know how often people actually want to commit to years of legal tussling just because Rush Limbaugh gets to be a huge, festering anus wart on the radio every day. It's a tough call. Maybe someone with more legal experience can chime in.