falsely accused at work

What is the name of your state? Tennessee
Actually this is about my husband. He is a plant manager and he does hiring and firing that sort of sturff. The other day he fired a person and the next day this person came to the plant and asked to talk to my husbands supervisor. He told them that my husband has been selling drugs at work and doing drugs at work and physically abused him. My husband was called into a meeting and was told this. Naturally it floored him. He does not do drugs nor sell them and is very critical of drugs. This person is retaliating by trying to get my husband fired. My husband told them to take him down for a drug test right now which they did. They also sent him home till results were in. The result were in and of course were negative but he is still not able to return to work because of the allegations made that are not even true. This is causing stress to him and I and we do not know what to do. There is no truth to this at all. His employers are behind him but have to do this. What legal grounds does he have to save his job? He is even afraid that if he does return to work it can happen again or someone can plant drugs in his desk or in his vehicle to get him out of there.

First of all, calm down.

When an allegation of illegal activity is made, the employer HAS to investigate. Employers are not always as stupid as some employees seem to think (I'm not talking about your husband; I'm talking about the employee making the allegation); most employers are quite capable of figuring out that when an allegation is made against a manager who has a clean record, by an employee who just got fired, that there's a strong likelihood that it's all a smoke screen/retaliation. But that does not remove their legal obligation to investigate the reported illegal behavior, even if they know going in that it's most likely all nonsense.

If his employers are standing behind him, there's no reason to assume yet that his job is in jeopardy. IF he loses his job (which from what you've said I think is quite unlikely, but IF) then he can very likely bring a lawsuit against the employee who made the allegations (though not against the employer). He might even want to talk to a local attorney to see if there are any local statutes that might help him.

Calming down has been a little difficult under the circumstances. When you try to do good and work hard to get where you are and what you have and someone can just blow your whole world apart.

Yes you are right, we may be jumping the gun here a little but its a scarey thing. My husbands employers are behind him. He is off of work right now but its a paid leave until this is resolved. They have a lawyer but it is for his behalf. His supervisor and others in the office were in tears over this. I realize they have to follow procedure but it is devasting and you ask yourself over and over why is this happening? They said it should take about a week for all this. They are questioning each employee. The one who started this whole thing even told them he comes to our house and smokes dope with my husband. He has never been to our house, I have never even met the guy. Its just crazy.
But we took ourselves for a long drive to distract us from this and it was nice.
Thanks