Is it legislated for a woman to observe a mourning period for any deceased person, male or female, even if there is no kinship between them, or are there specific regulations for that?

It is permissible to mourn for anyone whose loss causes sorrow, even if they are not a relative, such as a neighbor, a scholar, or an in-law. This is based on the general statement of the Prophet, peace and blessings be upon him: "It is not lawful for a woman who believes in Allah and the Last Day to mourn for a deceased person for more than three days, except for her husband, for whom she should mourn for four months and ten days." This Hadith (definition) includes both relatives and non-relatives. It seems that a friend is like a relative, as are a scholar and a righteous person. The criterion is anyone whose death causes sorrow; in such a case, she may mourn for three days.

Evidence for this is the weeping of the women of Banu Abdul-Ashhal for their fallen on the Day of Uhud, and then their weeping for Hamza, may Allah be pleased with him. However, it is preferable to avoid mourning to avoid disagreement, and some Shafi’is prohibit it. The apparent opinion of the Hanafis restricts it to relatives. If a woman wishes to mourn, it should be in a situation free from suspicion, and if her husband forbids her, she must refrain.