Talk to the parents and let them know you are concerned, both about the flakey-ness and that her hair is staying up for days at a time. They may not understand that it is important to take care of her hair. Let them know you love doing her hair, but that ponytails, braids, barrettes, etc should be taken out before she goes to bed (if nothing else, it could cause damage and breakage to the hair). Then show them her scalp and let them know that this is not normal for a child her age, and that it could be a medical issue (don't tell them you think it's because they aren't taking care of her hair, they'll go on the defensive!) and you think it might be wise for her to be checked by a doctor, just to rule out anything (and then the doc can suggest the hygene part if necessary)
Don't wash her hair more than every other day; too much washing could cause overstimulation of the oil glands, making the problem worse. Go ahead and keep an eye out for lice (as one poster suggested), comb through the back of her hair, lifting it in layers all the way down to the base of her neck, and look at the scalp for bugs - they aren't too hard to see if you get the layers of hair separated and can see to the scalp. But lice prefer clean hair and scalp, so I doubt they are hiding under the cradle cap.