What is the ruling regarding a mother who registered homes purchased by the father with his own money in her name, and refuses to return them to him, and now wants to register one of them in the name of a minor grandchild?

It is not permissible for a wife to register the house her husband authorized her to purchase in her name or in anyone else's name without his permission, because this is an encroachment on his property and a betrayal of trust. For an agent may only act within the limits of the principal's permission. Even if the husband permits her to register it in her name as a symbolic registration, without a transfer of ownership that fulfills the Sharia (definition) conditions for a gift, this does not make her the legal owner of the house.