So far your Dad bought $100 worth of bitcoin at the time, there will be private key for the public key or address used. You father will know that, because without the file or the private key backup, it may remian impossible for the bitcoin to be recovered. So, at the time, the address your father used to receive the bitcoin has private key which your father should know and appropriately backup like on paper. So, you will ask your father about the private key, where he backup the private key.