There is a piece of legislation called the Protected Disclosures Act which protects the interests of so called "whistl blowers" and is particularly relevant to criminal activities such as tax evasion.
If an employer dismisses an employee in contravention of the Protected Disclosures Act then this may constitute an unfair dismissal in terms of the Labour Relations Act. This would then require the intervention of the CCMA.
If you are interested in persuing the matter further then you may contact Labour Protect to assist you, likewise if you are already a member.