When a woman stops her menstrual cycle any time after the entry of Fajr, her fast that day does not count, and she has to make up (Qada) for that day. There is a consensus on this issue as stated by al-‘Allamah Ibn ‘Uthaymin (r) and as it is cited in al-Mawsu’ah al-Fiqhiyyah al-Kuwaytiyyah.
Shaykhu-l Islam, Ibn Qudamah (r) said:
“If one is menstruating during any part of the day, their fast is invalid, regardless if it was in the earlier part of the day or the latter.”
Now, as her fast that day isn’t valid, is she then allowed to eat and drink and do things that are permissible when one is not fasting? – There is Khilaf on this issue. See below:
The view of the Hanabilah is that she still has to refrain from things that are impermissible for a fasting person, like eating, drinking, engaging in sexual intercourse etc, due to the sanctity of the month. This, in Fiqh, is called “Imsak”.
However, many people seem to confuse “Imsak” for fasting. Their confusion leads them to believe that, as the female is refraining from things impermissible for the fasting person then that means she is also fasting, thus, she does not have to make the fast up. This, as you can see, is a clear blunder on the part of those who mix up the linguistic meaning of “Siyam”, which means to refrain in general (Imsak), and the technical “Imsak” which is to refrain from eating and drinking in certain scenarios (like the female who stopped menstruating after Fajr). So to repeat, this “Imsak” is a level below “fasting” and to translate it as “fasting” in books of Fiqh would be an error also and should be translated as “refraining” etc, so people don’t get confused.
Female stops menstruating after Fajr:
a) She still has to refrain (do Imsak) from all the things a fasting person refrains from.
b) This act of refraining (Imsak) does not mean that she does not have to make up for this fast, rather, she is just refraining because of the sanctity of the month.