We have previously discussed the difference of opinion among the scholars concerning the maximum length of nifaas; the most correct view is that it is forty days. See the answer to question no. 10488.
With regard to the bleeding that occurs after the period of nifaas has ended, if it comes at the usual time of the menstrual period, then it is menses and the woman should not pray or fast or have intercourse with her husband at that time, until her monthly period has ended, as is well known.
If it occurs at a time other than the usual time of the menstrual period, then it is istihaadah (non-menstrual bleeding) and in this case she should fast and pray, and her husband may have intercourse with her. But she has to do wudoo’ for each obligatory prayer after the time for it begins, then with that wudoo’ she may offer whatever naafil prayers she wishes.
For more information please see the answer to question no. 106464
The scholars (may Allah have mercy on them) differed concerning the case of a woman suffering istihaadah if she refrains from praying out of ignorance: does she have to make up the prayers that she omitted? There are two opinions:
1.That she has to make up the prayers.
2.That she does not have to make up the prayers. This is the view favoured by Shaykh al-Islam (may Allah have mercy on him).
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: If the woman who is suffering istihaadah did not pray for some time because she thought that it was not obligatory for her to pray, there are two opinions as to whether it is obligatory for her to make up the prayers. One opinion is that she does not have to make them up – as was narrated from Maalik and others – because when the woman who was suffering from istihaadah said to the Prophet (blessings and peace of Allah be upon him), “I have been bleeding heavily for some time and it prevents me from praying and fasting,” he told her what she should do in the future, and he did not tell her to make up the prayers of the past. End quote from Majmoo‘ al-Fataawa, 21/102
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: It is best if you pray what you omitted during the early days, but if you do not do that there is nothing wrong with it, because the Prophet (blessings and peace of Allah be upon him) told the woman who was suffering from istihaadah and who said that she was suffering heavy bleeding during which she did not pray to count her menses as six or seven days and to pray the rest of the month, but he did not tell her to make up the prayers that she had omitted. If she does make up the prayers that she missed, this is good, because it may be that she was heedless in not asking, but if she does not make them up there is no sin on her. End quote from Majmoo‘ Fataawa Ibn ‘Uthaymeen, 11/276
To be on the safe side, your friend should make up the prayers that she missed as much as she is able to. So every day she should make up whatever she can of the prayers that she omitted during this time, because it seems that there was a kind of heedlessness on her part as she did not ask even though the time during which she did not pray was lengthy, and usually one does not stop praying for so long. Moreover, she used to pray sometimes, which indicates that perhaps she knew that she should be praying. See also the answer to question no. 31803.
And Allah knows best.