If he/she has no proof that he/she was victimized by a former employee, then the company can very well go after that customer. We have presumptions in law:
"(d)That a person takes ordinary care of his concerns;
(q)That the ordinary course of business has been followed"
Following these presumptions, the burden is shifted to the customer to prove payment because a person who takes ordinary care of his concerns would ask for a receipt and that in the ordinary course of business a receipt is always issued. The best evidence to prove payment is the receipt and if the customer has none a presumption arises that is he/she has not paid. Of course, she can use other evidence to prove payment and overcome the presumption like the testimony of the former employee that he/she has paid.