Do you think they could be paying it too much attention?
Some times when kids see they are gettin attention for behaviors- even if its negative- they will continue it.
Maybe they can try a positive approach and try not to make a big deal out of it. ( if they have not already)
Try The all popular praise/ reward system and only Positive reinforcement - negative reinforcement might only undo all your hard work, and could confuse the child as to why she is being punished- she may not know where or when to go.
I do not know this little girl so I may just be rambling. This approach worked for my twins, it may work for her.
I understand the frustration- my now 9 year old had problems when he was around 4 potty training- I just don't remember what changed. I think he just grew out of it, and we had to deal with it.
She will get it. I know it don't seem like it now, but it will happen for her and her parents. Tell them they are doing awesome and to stay strong;) that goes for you too nanny!