I've never had this happen to me, but I have seen similar things happen in my news feed! For example, I have a small handful of "friends" on Facebook who will openly accuse and mock their landlord(s), "friends", employers and customers, etc. Because they have so much negativity to spread about everyone, I don't think many take much time to think about any one person that they choose to target. Let's just say, they are quick to make themselves look bad by being so negative.
That being said, do you feel like the person who targeted you could be the same way? Maybe they do this sort of thing all the time, and it's really hurting their own reputation more than anything? It might be best to make one public statement about the false review, but be careful not to get into a public brawl over it because it's just not worth it!
Instead of feeding into their trouble, simply try to fix anything on your part that could have added to the issue, but in all reality, remember that others can see straight through people who spread false lies, like your employee had done.
Do be careful not to appear defeated! Keep your head up and keep doing a great job for your other employees and this will pass!