You have witnessed a staff member from your host organisation accept a bribe from a rich businessman. What is your first course of action?
Quit your job immediately. Your government or organisation should not be seen as supporting this type of unethical behaviour.
Report the incident directly to your host government/ organisation. They need to know what is going on.
Make enquiries to verify your observations and look for additional proof and/or information.
Confront the staff member directly.