That sounds a LOT like prosopagnosia, but I'm no doctor. I've definitely thought a person looked like someone else I know (or even a celebrity) until I've seen them enough times that whatever feature(s) I thought looked like them don't seem so close anymore.
Perhaps you guys can try this Google search. I'd also look at "celebrity look-alikes" where it's obvious to you that it's two different people, but perhaps it might not be to her.