Can someone be sent to rehab without any actual evidence of drug use?

My room mate was just recently accused of smoking marijuana by a co-worker. I know for a fact that she does not smoke. She was suspended from her job and her insurance company wants her to go to rehab to make sure she gets her job back. She hasn't even officially lost her job yet. And Im just wondering if they can really send her to rehab without any proof that she smokes marijuana. She insists that they can, but that seems really unfair to me.