According to the employee handbook, contract, or past practice, inform the most relevant authority(ies) about specific behaviors witnessed.

Follow handbook, contract, or past practice concerning handling the potential dishonest behavior.

Don't immediately assume that the coworker is guilty of dishonest behavior. Use appropriate compliance hotlines if available.

Contact human resources for compliance help.

Don't spread the potential dishonest activity news to employees or others who do not have responsibility over the matter.