All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Whoever commits sins no matter how grave and serious they are, and then returns towards Allah, repenting to Him sincerely, then surely Allah will forgive him if he repents before death comes to him. Allah Says concerning this matter (what means): {Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."} [Quran 39: 53]
Allah also Says (what means): {The repentance accepted by Allah is only for those who do wrong in ignorance [or carelessness] and then repent soon after. It is those to whom Allah will turn in forgiveness, and Allah is ever Knowing and Wise.} [Quran 4: 17]
Whoever commits a sin is actually committing it in ignorance since he ignores the status of Allah when committing the sin.
The repentance of such a person is acceptable as long as he repents before his/her soul reaches his/her throat.
The Prophet said: “Allah accepts repentance of everyone as long as he is not facing the agony of death." Sometimes Satan tempts someone making him believe that Allah will not forgive him for the abundance of his sins and makes him feel beyond the reach of Allah's Mercy. Being hopeless of the Mercy of Allah is a grave sin. Moreover, it is a trick of Satan to have a person give up hope of the Mercy of Allah so he can drive him to commit endless sins.
One should know that Allah forgives all sins when the person makes sincere repentance; not only He forgives all sins, but He may even convert all his bad deeds into good deeds. This is only by His Mercy on His slaves and to humiliate Satan and to make him sorrowful.
Allah Says (what means): {And those who do not invoke with Allah another deity or kill the soul which Allah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse. And whoever should do that will meet a penalty. Multiplied for him is the punishment on the Day of Resurrection, and he will abide therein humiliated. Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good. And ever is Allah Forgiving and Merciful.} [Quran 25: 68-70]
As for making up the missed fasts and prayers, there are three reasons why she neglected them.
1. She neglected the prayers and fasts due to lack of knowledge that they are obligations, or because she supposed that they become compulsory at a certain age; in that case she is not obliged to make them up.
2. She neglected performing these acts denying that they are obligatory. Denying these obligations means apostasy from Islam, so she is not obliged to make them up.
3. She neglected praying and fasting intentionally after having knowledge that they are obligations.
In the latter case, the majority of the Muslim scholars are of the opinion that she has to make them up. Some others are of the opinion that she is not obliged to make them up. So, she should practice supererogatory prayers, fasts, give charity, and all kinds of good deeds as much as possible. She should try to be in the company of good Muslim ladies who will help her in doing good and avoiding evil.
We ask Allah to guide her to what pleases him and make her steadfast on the right path.
Allah knows best.