From my experience violence escalates. If he has hit her once, chances are he may hit her again. After all, there was no consequences to his actions. Of course, she will become even more careful of what she says and does. I think you have every right to be worried. I would offer to go with her to her doctor's appointment and go over it with him. She will probably not want that but I would try to insist. Of course, your dad has to be willing to change and that will be the hard part because he may not see he has a problem.
Good luck
Gayle