Final answer:
The Messenger should tell Medea that by taking revenge and inflicting harm on Jason, she will feel twice the pleasure.
Step-by-step explanation:
The Messenger should tell Medea that by taking revenge and inflicting harm on Jason, she will feel twice the pleasure. By causing Jason to suffer for his actions, Medea will experience a sense of satisfaction and vindication, which will bring her pleasure. The Messenger can emphasize to Medea that her revenge will not only serve as a punishment for Jason, but also as a source of personal satisfaction for herself.