Her period lasts all month because of a weakness in her reproductive glands
Question

                There is a woman in my family who has her period all month because she suffers from a weakness in her reproductive glands. Now she is undergoing treatment but that happens to have coincided with Ramadaan. She is wondering what she should do –should she fast?.
Praise be to Allah.
    If the bleeding lasts all month then she is experiencing 
    istihaadah (irregular non-menstrual vaginal bleeding) and she should refer 
    to her regular cycle if she used to have a regular cycle before that, and 
    stop praying etc during the time that was her regular period, then she 
    should do ghusl and pray and fast for the rest of the month even if she is 
    still bleeding. If she did not have a regular cycle or she has forgotten it, 
    then she should distinguish the types of blood if she can. Menstrual blood 
    (hayd) and non-menstrual blood (istihaadah) may be distinguished by colour, 
    smell and thickness; menstrual blood is black or dark, and has an unpleasant 
    smell, and it is thick, unlike the blood of istihaadah. 

    The days on which the blood has the characteristics of 
    menstrual blood are menses, and on the other days she is regarded as “pure”, 
    so she should pray and fast. 

    If she cannot distinguish different types of blood, then she 
    should stop praying etc for six or seven days, because that is the usual 
    length of menses for women, then she should do ghusl and pray and fast. 

    Among the reports of the Sunnah which speak of istihaadah and 
    say that the woman should refer to her previous cycle – if she used to have 
    a known cycle – is the report narrated by al-Bukhaari (319) from ‘Aa’ishah 
    (may Allaah be pleased with her), that Faatimah bint Abi Hubaysh asked the 
    Prophet  (peace and blessings of Allaah be upon him): I suffer from 
    istihaadah and I do not become pure; should I stop praying? He said: No, 
    that is just a vein. But stop praying for the number of days that you used 
    to menstruate, then do ghusl and pray.” 

    Among the reports that speak of differentiating between types 
    of blood is that narrated by al-Nasaa'i (215) and Abu Dawood (304) from 
    Faatimah bint Abi Hubaysh, that she suffered from istihaadah and the 
    Messenger of Allaah  (peace and blessings of Allaah be upon him) said to 
    her: “If it is menstrual blood then it is black and recognizable, so refrain 
    from praying, then when it is the other type, do wudoo’, for that is from a 
    vein.” Classed as saheeh by al-Albaani in Saheeh al-Nasaa’i. 

    Among the reports which speak of the woman who has istihaadah 
    stopping prayer for six or seven days if she did not have a regular cycle or 
    she cannot distinguish between types of blood is the report narrated by 
    al-Tirmidhi (128) and Abu Dawood (287) from Hamnah bint Jahsh who said: I 
    used to suffer istihaadah, bleeding heavily. I came to the Prophet  
    (peace and blessings of Allaah be upon him) to ask him and tell him about 
    that. I said: O Messenger of Allaah, I suffer istihaadah, bleeding heavily. 
    What do you command me to do? It is keeping me from fasting and praying. He 
    said: “That is only a kick from the shaytaan, so 
    count your menses as six or seven days, which is something between you and 
    Allaah, then wash yourself and when you see that you have become pure and 
    you are certain of it, then pray for twenty-four or twenty-three days, and 
    fast and pray. That will suffice you, so do what other women do with 
    regard to their menses and purification.” 

    Al-Tirmidhi said: I asked Muhammad [al-Bukhaari] about this 
    hadeeth and he said:  It is a hasan saheeh hadeeth. Ahmad ibn Hanbal said 
    likewise, it is a hasan saheeh hadeeth. 

    And Allaah knows best.