Timmin was Morgus' personal assistant and thus privy to all his illegal dealings. She had apparently worked for Morgus for a long time, and she had come to hate him like many other people.
After Morgus departed for Androzani Minor, she betrayed him to the government, becoming the new chairman of the Sirius Conglomerate. She told the government all of Morgus' crimes, giving them information about how he'd arranged for the murder of the President, embezzlement and fraud. (TV: The Caves of Androzani)