There is a difference of opinion in the matter. If the wife was pressured into intercourse, or committed the act out of forgetfulness, or did not possess the knowledge that it is forbidden to do so while fasting in Ramadan, then her fast would be valid and she would not have to make up her day or offer expiation.
On the authority of ibn ‘Abbas (ra), the Prophet (pbuh) said, “Verily Allah has pardoned [or been lenient with] my ummah: their mistakes, their forgetfulness, and that which they have been forced to do under duress.”
If the woman does not have a valid excuse and complied with her husband in having intercourse, then she must make up the day. As for the requirement of offering expiation, the scholars have differed:
First opinion: She must make up the day and offer expiation if she was complaint with that act; that is the opinion of the majority. Evidence for this is the sahih hadith previously mentioned that stated the command of the Prophet (pbuh) to the man who had intercourse with his wife to offer expiation. The general rule is that the same rules apply to men and women alike unless a specific difference is stated.
Second opinion: She must make up the day; however, the expiation is only for the husband. This is the case whether she was coerced or not. This is the view of the Shafa’i school and a report from Imam Ahmad. Evidence for this view is that the Prophet (pbuh) ordered the man to offer expiation and did not mention the woman. Therefore, they stated that it is not permissible for the Prophet (pbuh) to delay the explanation of a ruling from its time of need.
The stronger opinion is that expiation is required for the woman just as it is required for the man.