Baby Conceived a Month Later?

Hi, my current partner's ex recently had a baby whose original due date was the 4th of December and is saying it is his. I have done some research and found that possible conception would have taken place mid march this is also logical with the due date. As I was with my partner at this time i know it is not possible also both my partner and his ex acknowledge the last time they had sex was mid February. We have tried to bring this up with her multiple times but she refuses to give us a straight logical answer. Is there at all a possibility that the baby is his ?