What is the ruling on a husband having intercourse with his wife while making up for missed fasts, having forgotten, when she was aware and coerced?

The wife erred by breaking the obligatory fast after having started it, which is not permissible for her except out of necessity. She is sinful and must repent and make up the day. As for the husband, there is no sin upon him because he was forgetful. There is no expiation (kaffarah) required for the wife because expiation is only necessitated by violating the sanctity of Ramadan.