What can I do about a neighbor who is harassing me?

My daughter lives in a third floor condo and the neighbor under her is complaining that she's walking on her floor too loudly and that her bathroom is leaking into her bathroom. She is going to get carpet and she had a plumber check her bathroom and they couldn't find anything. In the mean time my daughter has found her windshield wipers bent, her mailbox keyhole filled with gum and packages she has ordered missing. What should my daughter do?