Praise be to Allah.

Firstly: 

If a person is illiterate and cannot read or write, then he must be taught what to say in prayer and how to pray. The fact that he cannot read or write does not mean that he cannot do that, because it is something that is easy, praise be to Allaah. Most of the Sahaabah (may Allaah be pleased with them) could not read or write, but they learned well how to pray. 

Reciting al-Faatihah in prayer is one of the pillars or essential parts of the prayer, whereas reciting a soorah after it is Sunnah and mustahabb, so if your mother does not do it, there is no sin on her. But you should help her to memorize some short soorahs such as al-Ikhlaas and al-Mi’wathatayn (i.e., the last three soorahs of the Qur'aan). 

With regard to sujood (prostration), what one must say in it is Subhaana Rabbiy al-A’la (Glory be to my Lord Most High) once, but it is mustahabb to say it at least three times. This phrase is undoubtedly easy, but if a person cannot say it that is due to his own shortcomings and negligence. 

Whatever the case, you should be kind to her and help her to carry on praying, and teach her whatever she needs to know of the dhikrs that are said in prayer. If she is unable to learn them, but she can recite al-Faatihah well as you say, then she can say whatever du’aa’s and tasbeehs she can, and whatever she is unable to do will be waived and she will be pardoned for that.

Abu Dawood (792) narrated that the Prophet (peace and blessings of Allaah be upon him) said to a man: “What do you say in your prayer?” He said: I recite tashahhud and say: O Allaah, I ask You for Paradise and I seek refuge with You from Hell. But I cannot murmur as well as you and Mu’aadh do. The Prophet (peace and blessings of Allaah be upon him) said: “That is what we are murmuring about.” Classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

Shaykh al-Islam said: The one who follows this principle, which is that the obligatory parts of prayer are waived for the one who is unable to do them, as are the obligatory parts of prayer in congregation and so on, has come to the right conclusion that is indicated by the Sunnah of the moderate way between the idea of neglecting some obligations of sharee’ah altogether and the idea of regarding them as not subject to being waived at all to the point that it makes a person ignore other obligations which are more important that that which he waived because he is unable to do it, even though he is able to fulfil these obligations that are more important. Therefore, to do what one is able to do and not do what one is unable to do is the middle way between two extremes. 

End quote from Majmoo’ al-Fataawa (23/247). 

Secondly: 

Denying the resurrection after death or doubting that it will happen is kufr which puts one beyond the pale of Islam according to scholarly consensus, because of the sound and clear evidence in the Qur'aan and Sunnah which speaks of the resurrection. Rather common sense and sound human nature also point to that.  

Allaah says (interpretation of the meaning): 

“The disbelievers pretend that they will never be resurrected (for the Account). Say (O Muhammad صلى الله عليه وسلم): Yes! By my Lord, you will certainly be resurrected, then you will be informed of (and recompensed for) what you did; and that is easy for Allaah”

[al-Taghaabun 64:7] 

“And indeed We created man (Adam) out of an extract of clay (water and earth).

13. Thereafter We made him (the offspring of Adam) as a Nutfah (mixed drops of the male and female sexual discharge and lodged it) in a safe lodging (womb of the woman).

14. Then We made the Nutfah into a clot (a piece of thick coagulated blood), then We made the clot into a little lump of flesh, then We made out of that little lump of flesh bones, then We clothed the bones with flesh, and then We brought it forth as another creation. So Blessed is Allaah, the Best of creators.

15. After that, surely, you will die.

16. Then (again), surely, you will be resurrected on the Day of Resurrection”

[al-Mu’minoon 23:12-16]

“And the Trumpet will be blown (i.e. the second blowing) and behold from the graves they will come out quickly to their Lord.

52. They will say: ‘Woe to us! Who has raised us up from our place of sleep.’ (It will be said to them): ‘This is what the Most Gracious (Allaah) had promised, and the Messengers spoke truth’”

[Yaa-Seen 36:51-52] 

“And he puts forth for Us a parable, and forgets his own creation. He says: ‘Who will give life to these bones after they are rotten and have become dust?’

79. Say: (O Muhammad صلى الله عليه وسلم) ‘He will give life to them Who created them for the first time! And He is the All‑Knower of every creation!’”

[Yaa-Seen 36:78-79]

And there are many other verses which speak of the resurrection and requital, followed by Paradise or Hell. And there are mutawaatir ahaadeeth which tell of how people will rise from their graves and be assembled in the place of gathering, and they will stand, barefoot, naked and uncircumcised, and the sun will be brought close to their heads, and there will be the reckoning and requital, and admittance to Paradise or Hell. 

These are things which no Muslim has any excuse for not knowing; rather even non-Muslims such as Jews and Christians know them, so the one who denies them is undoubtedly a kaafir. 

What you must do is explain to her how Allaah will honour those who obey Him, and will make the reckoning easy for them and admit them to His Paradise and the abode of His honour, and tell her about the punishment and humiliation that He has prepared for those who disobey Him and go against Him, and their miserable state at the time of death, in the grave and on the Day of Resurrection. 

Tell her of Allaah’s perfect justice, wisdom and mercy, and that it is not befitting for Him to leave mankind with no resurrection or reckoning: 

“ ‘Did you think that We had created you in play (without any purpose), and that you would not be brought back to Us?’

116. So Exalted be Allaah, the True King: Laa ilaaha illa Huwa (none has the right to be worshipped but He), the Lord of the Supreme Throne!

117. And whoever invokes (or worships), besides Allaah, any other ilaah (god), of whom he has no proof; then his reckoning is only with his Lord. Surely, Al‑Kaafiroon (the disbelievers in Allaah and in the Oneness of Allaah, polytheists, pagans, idolaters) will not be successful.

118. And say (O Muhammad صلى الله عليه وسلم): ‘My Lord! Forgive and have mercy, for You are the Best of those who show mercy!’”

[al-Mu’minoon 23:115-118]

And it is not befitting for Him ultimately to treat believers and disbelievers, righteous and evildoers, equally. 

“Shall We treat those who believe (in the Oneness of Allaah __ Islamic Monotheism) and do righteous good deeds asMufsidoon (those who associate partners in worship with Allaah and commit crimes) on earth? Or shall We treat the Muttaqoon (the pious) as the Fujjaar (criminals, disbelievers, the wicked)?”

[Saad 38:28] 

“Or do those who earn evil deeds think that We shall hold them equal with those who believe (in the Oneness of Allaah __ Islamic Monotheism) and do righteous good deeds, in their present life and after their death? Worst is the judgement that they make”

[al-Jaathiyah 45:21]

Tell her that these ideas are a slander against Allaah and disbelief in Him. Al-Bukhaari (3193) narrated that Abu Hurayrah (may Allaah be pleased with him) said: The Prophet (peace and blessings of Allaah be upon him) said: “The son of Adam reviles Me and he has no right to revile Me, and he denies Me and he has no right to do so. As for his reviling Me, it is his saying that I have a son. And as for his denying Me, it is his saying: He will not remake me as He made me at first.” 

Seek help in teaching and guiding her from those whom you trust among her children and relatives. 

We ask Allaah to guide her and set her affairs straight. 

And Allaah knows best.