To stop him from leaving, the girl proceeds on lying to him about being pregnant. The lie lasted for only 3 months and obviously he dumps her for good. What's the point of lying over something so obvious then he's going to leave her anyway?
Secondly, even if it was a real pregnancy why would any woman want a man that's only with her because of a baby, knowing very well he wanted to end things... basically only staying with her out of pity and for the baby?