She is eighty years old and makes mistakes in reciting Qur’an, and she forgets; what is the ruling on her prayer?

Firstly: 

Reciting al-Faatihah is one of the pillars or essential parts
of the prayer, which the worshipper must recite correctly. The one who is
excused for not doing that properly, because of a speech defect,
forgetfulness and so on, must recite it to the best of his ability, and
whatever he is unable to do is waived in his case, because Allah, may He be
exalted, says (interpretation of the meaning):

“On no soul does
Allah place a burden greater than it can bear”

[al-Baqarah 2:286]. 

The prayer is not rendered invalid unless the worshipper
omits something from al-Faatihah or changes the pronunciation in a manner
that alters the meaning. This applies if a person is able to recite it
properly. In the case of one who is not able to do that, he should recite it
as best as he can. 

If a Muslim is not able to offer the prayer in the perfect
manner, then he must do whatever he is able to do, and whatever he is unable
to do is waived in his case. 

Al-Bukhaari (7822) and Muslim (1337) narrated that Abu
Hurayrah (may Allah be pleased with him) said: The Messenger of Allah
(blessings and peace of Allah be upon him) said: “If
I command you to do something, then do as much of it as you can. 

An-Nawawi (may Allah have mercy on him) said: 

This is one of the most important basic principles of Islam,
and is an example of the power of concise speech that was given to the
Prophet (blessings and peace of Allah be upon him). It is applicable to
innumerable rulings, such as prayer of all types. If a person is unable to
do some of the pillars or essential parts of the prayer, or is unable to
meet some of its conditions, then he must do the rest; if he is unable to
wash some of the parts of the body that must be washed in wudoo’ or ghusl,
then he must wash whatever he can. If he has something that will cover part
of his ‘awrah, or he has memorised only part of al-Faatihah, then he must do
whatever he is able to do. … This hadeeth is in accordance with the words of
Allah, may He be exalted (interpretation of the meaning):

“So keep
your duty to Allah and fear Him as much as you can”

[al-Taghaabun
64:16].

End quote. 

See also the answer to question no.
5410 

What you must do is focus on teaching your aunt to recite
al-Faatihah correctly, if she is not reciting it properly and is able to
learn it. 

Secondly: 

With regard to soorahs other than al-Faatihah, the matter is
easier, especially when one is not able to do it, because reciting something
other than al-Faatihah in the prayer is not one of the obligatory parts of
the prayer. The scholars of the Standing Committee said: 

If the imam forgets a verse of al-Faatihah and doesn’t
realise it until a long time has passed, then he must repeat the prayer if
it was an obligatory prayer, because recitation of al-Faatihah is one of the
pillars or essential parts of the prayer. But if he remembers that before a
long time has passed, then he should do an extra rak‘ah to replace the
rak‘ah in which he omitted a verse of al-Faatihah, then do the prostration
of forgetfulness (sujood as-sahw). 

But if the verse that was forgotten was from a soorah other
than al-Faatihah, then his prayer is valid and neither he nor those who
prayed behind him have to do anything, because recitation of more than
al-Faatihah is mustahabb (encouraged) but is not obligatory.

End quote from Fataawa al-Lajnah ad-Daa’imah, 5/332 

Shaykh Ibn Baaz (may Allah have mercy on him) said: 

With regard to recitation other than al-Faatihah, if the
worshipper omits anything because he forgot, that does not matter. 

End quote from Fataawa Noor ‘ala ad-Darb, 9/421 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was
asked: 

My mother has high blood pressure, as a result of which she
is half-paralysed and cannot move the muscles on the right side of her body.
Her memory has also become weak, and she has forgotten some verses of the
Qur’an, including al-Faatihah, as well as some of the adhkaar that are
recited in the prayer. Her speech is also affected, in that she cannot speak
except with great effort. My question is: What is the ruling on my mother’s
prayer if she forgets some verses or al-Faatihah or some of the obligatory
adhkaar of the prayer, after she does her best? 

He replied: 

If she cannot do more than that, then she is excused, because
Allah, may He be exalted, says (interpretation of the meaning):

“So keep
your duty to Allah and fear Him as much as you can”

[al-Taghaabun
64:16].

“On no soul does
Allah place a burden greater than it can bear”

[al-Baqarah 2:286].

But she should strive her utmost to recite al-Faatihah and
the obligatory adhkaar to the best of her ability, even if that means that
someone should be with her who can prompt her. With regard to things that
are mustahabb, such as reciting more than al-Faatihah or more than
Subhaana Rabbiy al-A‘la (Glory be to my Lord Most High) when prostrating
and Subhaana Rabbiy al-‘Azeem (Glory be to my Lord the Almighty) when
bowing, and so on, there is nothing wrong with omitting that.

End quote from Fataawa Noor ‘ala ad-Darb, 8/2. 

And Allah knows best.