Is it permissible for a woman to observe a mourning period (إحداد) for a deceased person with whom she has no kinship ties, such as her neighbor, or are there specific regulations for this?

It is permissible for a woman to mourn for a deceased person other than her husband for three days, if she grieves for his death, even if he was not a relative. This is supported by the hadith of Umm Habibah, may Allah be pleased with her, that the Prophet, peace and blessings be upon him, said: "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 four months and ten [days]."