You never know what's going througha dog's mind.

Sometime when your dog isn't with you, but your Dad a box of its favorite treats. Then when you go over, have your Dad each time (if he'll cooperate), crouch down and hold out a treat, so he's not so big and threatening, and is gradually seen as a source of good things. It's bribery, but I've seen it work. She may never "like" him, but you can get it to the point where she's not afraid. Tell Dad also to avoid eye contact with the fraidy-dog, and have him ignore the dog afterwards until the dog approaches him.