Big plot hole

That bothered me, too. I suppose we are asked to presume that Mr. Courtland gave them the night off(?).

Not a plot hole. We don't see it happen, but the most plausible explanation is that Alison's husband set it up so that she'd be home alone. Is it a plot hole that we don't literally see Alison's husband making the appointment with the pretend Dr. Rhinehart? It's not like the filmmakers forgot about the house staff - the imposter is seen knocking for a very long time at the door and Alison looks around for the butler (calling out his name) before she answers the door.