It IS slander, but her boss fell into his own trap... She needs to relax...
If, of course, she was selling drugs, the reasonable thing for an employer to do is fire her and call the cops. This guy didn't fire her. He didn't even fire her AFTER he made the accusation. Those actions PROVE that he made it all up, and slandered her.
However, in order to WIN a slander lawsuit, the plaintiff has to show damages. Here's the kicker. As long as she stays employed, she has no damages. If she gets FIRED, she'd have BIG damages. So, it looks to me, that because she WAS slandered, her job is GUARANTEED.