Someone is making a false accusation about a friend of mine, can he take legal action?

My friend has been falsely accused of taking a bottle of medication from a function he attended. The reason they have blamed him is because as a nurse they believe he was the only person at the function that would have any knowledge of the fact that that medication is highly sought after by recreational drug users. There is no other 'evidence' to suggest that he took it. He states that as a nurse he knows better than to take any type of medication that is not prescribed specifically for him and knows better than to use illicit/recreational drugs. He is willing to undergo testing to prove he hasn't taken it. He would like to take legal action because this situation has caused him much stress and implies things about him that are not true. He is concerned because as a nurse his reputation and career could be destroyed with this sort of allegation. Is there any thing he can do?Someone is making a false accusation about a friend of mine, can he take legal action?
it is part of the job..I have worked in a vet clinic for over 10 years and part of the job is to fill medications prescribed by the vet...if anything goes missing in lock up (controlled substances) the doctors and nurses are first on the list of suspision. drug testing can rule our personal usage..but it doesnt rule out selling or giving away the drugs...so integrity, loyalty and intelligence is a must in order to establish and maintain your demeanor in the workplace. I dont think he has any sort of lawsuit..if he does not like the way he is treated at work..he can notify and complain to managment or simply find new employment