My friend is cutting herself, what should I do?

Today during P. E. everyone was having fun and playing basket ball when all of a sudden I saw something on my friends arm. Asking to see it, she wouldn’t show me, so I pryed her arm away from her body and got a good look at at least 10 cuts. I asked her what happened and she said nothing. No matter how many times I ask what’s wrong or what happened, she say it’s nothing and she’s fine. What should I do? Should I tell someone, or let it go? With her being my best friend, I feel like I should help her somehow.