i believe the mangement should be highly diplomatic in dealing such spies. its much better the management should undertake bargain with the spy on subject he had assessesed the information about the company. the management should persuade him and compensate him for retaining the information. this sounds little awkward but still he should be paid the value. the management should maintain confidentiality regarding the compensation paid to the spy.
i think the first step will be to summon the employee for questioning. we need to find out the motive behind his actions, perhaps a quest for information that will help his company or on a personal venture. i believe whichever is the case, he must not necessarily be fired; he can be given a benefit of doubt by cautionary measure.