Wrongful Termination

seek an advice for a friend who was terminated from his organization. He works with a public listed multinational. He and one of his colleagues were detained in the office one day and other employees were sent home and asked to work from home. They were questioned for 8 hrs by a third party with bouncers in office. His colleague was terminated and my friend was sent home. He was not given any chance to explain nor was any management team organized to conduct a formal investigation. After that session he went in the office after that humiliation my friend decided not to go to that office and he sent his resignation. The company did not sent any acknowledgment or acceptance for more than 8 days and when he followed up they send him a termination letter stating that he was suspected of fraud. My friend sent an follow up email saying that he does not accept any allegation. Please advise what his rights are in orders to get his termination revoked as it impacts his career and also limits his changes for employment at a good company. Can a company conduct an investigation this way. What should he do when a company does not respond to his non acceptance of termination. Why are they not reply to his follow up emails?? Any suspected strategy?? Please advise what he can do in this case