My friend self-harms? Help please?

Okay, so my friend (let's call her J) self-harms. I only found out today. She few her nails really long, and said to everyone that she just scratched herself. When I got a clear look at the scars, it was obvious that it had been done with a blade. I questioned her, and asked her 'When did you start cutting?' She just responded, 'Since the beginning of the end,' in a joking voice. She's a very fun person who doesn't take a lot of things seriously. It's very hard to get her to answer seriously. I want to get through to her, but she won't take it seriously. How can I get her to answer me?