Optimum strategy is to go to HR and tell them "listen - this guy have sexual harassment charges against him. Now he want me to convince the ex-employee to drop the charges. AND if I won't do that he threaten me with firing me. So not only HE will get two charges, in which one will be obstruction of justice, but THE COMPANY will get third one for wrongful dismissal."
HR are there to protect company and this "boss" is doing everything to make the company looks bad. This is not good PR and if this case will make just slightly looks like company is covering for sexual offender the ship will hit the fan faster than law team will be able to write tweet with apologies.