Praise be to Allah.
The sick person who has a concession allowing him (or her) not to fast should realize that it is makrooh to fast if it is too difficult for him, and it is haraam if it will harm him. Allaah has granted him a concession allowing him not to fast, so it is not permissible for him to impose hardship upon himself, and it is not permissible for him to cause harm to himself.
Swallowing blood is one of the things that invalidate the fast, but if some blood enters a person’s throat involuntarily, without him intending that to happen, then there is no sin on him and he does not invalidate his fast thereby, but if he deliberately swallows it then his fast is invalidated thereby.
Ibn Qudaamah (may Allaah have mercy on him) said:
If his mouth bleeds and he swallows it, then he has broken the fast, even if it is a small amount, because the mouth comes under same ruling as external things, and the basic principle is that the fast is broken by everything that comes from the outside. But saliva is exempted from that because it is not possible to avoid it. But everything else remains subject to this principle. If he spits it out from his mouth and his mouth remains impure or his mouth becomes impure with something from outside, and he swallows his saliva, if there is any part of the impurity with it then his fast is broken because of that, otherwise it is not. End quote.
The scholars of the Standing Committee said:
If there are ulcers on his gum or the miswak makes them bleed, it is not permissible to swallow the blood, and he has to spit it out. But if it enters his throat without him meaning it to, there is no sin on him. Similarly in the case of vomit, if it goes back into his throat without him meaning it to, then his fast is still valid. End quote.
Fataawa al-Lajnah al-Daa’imah (10/254).
Shaykh Muhammad al-Saalih al-‘Uthaymeen (may Allaah have mercy on him) was asked: Does a person invalidate his fast if he bleeds when he has a tooth pulled out?
Bleeding when having a tooth pulled out does not affect the fasting person at all, but he must avoid swallowing the blood, because the blood is something external and extraordinary, and swallowing it breaks the fast, unlike swallowing saliva which does not break the fast. The fasting person who has a tooth pulled out must be careful to avoid letting the blood reach his stomach, because that breaks the fast. But if the blood enters his stomach accidentally, then it does not matter, because he did not do that deliberately. End quote.
Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen (19/question no. 213).
The Shaykh (may Allaah have mercy on him) also said:
If a person has a nosebleed and some of the blood reaches his throat and some of it comes out, that does not break his fast, because that which reached his throat did so without him meaning it to, and that which came out does not affect him. End quote.
Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen (19/question no. 328).
It is mustahabb for her not to fast if fasting is too difficult for her, and it is obligatory for her not to fast if fasting will harm her. If she does not fast, then she has to make up the missed days if she is able to. If she is not able to make them up, then she has to offer the fidyah which is feeding one poor person for each day.
We ask Allaah, the Lord of the Worlds, to decree reward for her for her patience, and to heal her sooner rather than later.
And Allaah knows best.