Is it permissible for a widowed woman to marry herself without a guardian, and is it permissible for a daughter to prevent her widowed mother from marrying?

According to the majority of scholars, a woman's marriage without a guardian (wali) is invalid. However, it is considered valid by Abu Hanifa. A marriage without a guardian can be enacted if a Sharia judge rules it valid, or if someone who follows Abu Hanifa's school of thought performs the marriage contract. A daughter does not have the right to prevent her mother from marrying, but she should advise her mother on the necessity of the guardian's consent, given the prevalence of this opinion among scholars.