How can I persuade my friend to wear sunscreen?

My friend and I are at swim camp, and basically spend the whole day out in the sun, but she last used sunscreen two weeks ago. I tried telling her what the risks are of not wearing sunscreen, but she JUST DOESN T LISTEN. She says that she doesn t get burnt, has a tab already, and thinks that her skin is in perfect condition, so there are no chances of her getting skin cancer or anything. She has a really REALLY bad tan, and I m just don t know what to do. HELP?!