A woman who has been married for twenty years vitiated her fast nine times during Ramadan—five times right after the wedding and four times in the following year. This was because she engaged in sexual intercourse with her husband during the fasting hours of Ramadan. She was ignorant of the prohibition of this act and did not know the gravity of the sin. When she later inquired into the matter, some people advised her to fast two months consecutively, while others told her that only her husband is blameworthy. When some people advised her to feed sixty poor persons [in expiation], her husband refused to either pay or fast. She would like to know what she and her husband must do.

If the case is as mentioned in the question, the inquirer and her husband must make up the missed fast days by each fasting nine days instead. In addition to this, the husband must make expiation because he transgressed the limits ordained by Allah. It consists of fasting two consecutive months for each day he has missed. If unable to fast some or all the days, he is to feed sixty indigent persons of the average food he feeds his family for each day he is unable to fast.It has been reported in an authentic hadith that the Prophet ordered the man who approached him for the ruling for vitiating his fast in Ramadan by engaging in sexual intercourse with his wife to make expiation; the Prophet did not order his wife to do likewise. This was an incident that made it necessary to clarify the ruling since it is not permissible to delay the clarification of a matter beyond its time. It therefore became known that a woman is such circumstances must only make up the missed fast days.
The above answers the question and Allah the Almighty knows best.