All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
There is no doubt that calling people to Allaah is a great act of worship and a matter that can get one closer to Allaah; many texts from the Quran and the Sunnah were reported about its excellence, and we mentioned some of them in Fatwa 85108.
Admonishing the people and guiding them to do good does not require much knowledge; rather, a Daa’iyah (caller to Allaah) should be good at what he wants to convey to the people even if it were only one verse or one Hadeeth. ‘Abdullaah Ibn ‘Amr narrated that the Prophet said: “Convey on my behalf even if it were only one verse (from the Quran)." [Al-Bukhari]
Also, the Prophet said: “May Allaah brighten (the face of) a person who hears my words, preserves them and then conveys them to those who had not heard them. At times the one carrying Fiqh (religious knowledge) has no Fiqh himself, and at times the one carrying Fiqh conveys it to one who has Fiqh more than himself.” [Ahmad, Abu Daawood, At-Tirmithi and Ibn Maajah]
Hence, this sister did well by being keen on doing Da’wah, spreading knowledge and guiding the people, but it is not permissible for her to handle a subject she has no knowledge of, especially issuing Fataawa as this is a very serious matter and daring to issue Fatwa (without knowledge) is like daring to be sent to the Fire. For more benefit on warning against issuing Fataawa, please refer to Fatwa 89691.
Therefore, we advise you to encourage this sister to spread good and virtue, and you should advise her in a good manner and privately as the objective is to advise her and not to rebuke her.
However, you may seek the help of her husband in this regard as he is one of the Dua’ah (callers to Allaah) and a seeker of religious knowledge. Also, you may seek the help of those who are in charge of the place where she gives lessons if she is holding such lessons in Islamic Centers and the like.
On the other hand, if what is mentioned about her is true that she is harsh and that she mistreats the sisters, then she should be reminded about the importance of having a good moral conduct especially for a person who calls to Allaah, and that she should be a good example.
If advising her and rectifying her are of no avail, and it is predominantly thought that her giving such lessons is dangerous for the sisters who are attending her lessons, then one should warn these sisters against her out of sincere advice to the Muslims. Tameem Ad-Daari narrated that the Prophet said: “Religion is (sincere) advice". They said, "For whom?'' He replied, "For Allaah, His Book, His Messenger and for the leaders of the Muslims and their masses." [Muslim]
Allaah Knows best.