My father married a woman,
and then a dispute arose between him and her family, so he divorced her. I then went to her family and proposed to her, but her father told me that it is not Halal (lawful) for me to marry her, because she was married to my father. I hope that Your Eminence will guide me to the lawful path regarding this matter.
If the situation is as you mentioned and your father had concluded a marriage contract with the woman, she was his wife, even if they did not consummate the marriage. (Part No. 18; Page No. 207) It is therefore Haram (prohibited) for you to marry her after your father has divorced her. Allah (Exalted be He) says (what means),
And marry not women whom your fathers married, except what has already passed; indeed it was shameful and most hateful, and an evil way.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.