My Granmother was the same way . I was the only 1 she trusted for many years . If your father tries finding someone to watch her and make sure she takes her pills etc , he might be able to convince her that that the person can be trusted as much as him . I found 1 to help me with my Granmother , she didn`t like or trust her the first few days but it changed since she found out I did . It might work if he tells your Granmother the helper tells him everything and he will be very dissapointed if she gives the helper a lot of trouble . That made my Granmother accept the woman I found to help out . It is a very difficoult situation to try to give advice in , but if your father can get someone to be trusted by your Granmother that would be the right way .