What is the ruling on a father preventing his daughter from visiting her mother because she refused to separate from her Hindu husband, with whom she had a child, after she ran away with him seven years ago?

It is not permissible for a Muslim man or woman to marry polytheists. Hindus are polytheists, and therefore, this marriage is religiously invalid. The father must advise his daughter and remind her of Allah and His punishment. The girl must fear Allah and leave this polytheist, or try to have him embrace Islam and then contract a valid Islamic marriage with her under its conditions, while repenting to Allah. As for the father preventing his daughter from visiting her mother, it is permissible if it serves as a deterrent to her; otherwise, it is not permissible because it severs kinship ties. Living in lands of disbelief is fraught with dangers and leads to the loss of children.