Dropping a class

Hello all,
D is a senior in HS, she was put on deferral at Villanova. She is a diligent student with excellent extracurriculars and a very strong GPA. One of her AP classes this year is giving her a hard time. She is considering withdrawing from the class. Is this frowned upon? She has a 75 in the class so I believe it would be better for her to drop rather than stick it though. How would colleges look at this, esp Villanova since it is her top? Advice?