As long as the accusation was made in public with other customers around it can be slander. In the UK it would be for the defendant (I.e. security guard/store)

As long as the accusation was made in public with other customers around it can be slander. In the UK it would be for the defendant (I.e. security guard/store) to prove that the claim was actually true. So in this case they would be stuffed. The person should definitely contact a solicitor.