I think my bestfriend is copying my self harming? :(?

I've been self harming for a year and a bit now. Only my closest best friend knew, but I invited a relatively new friend over for a sleepover, so I went to get changed in the bathroom (because I didn't want her to see my arms) and I just told her I needed to sort something out and when I was changing tops she burst in the bathroom laughing about something and she started saying something about my brother and then stopped and freaked out and went''Oh my god what the f*ck have you done'' I just panicked and grabbed a towel and covered up. I explained to her why I did it but she just got angry and said I was stupid. I told her she cant tell anyone etc. Eventually she let it go but it was awkward all night. The next day on msn I just explained all my feelings and why I started and how it helps and she said she understood and she was there for me. So I invited her over again about a week later and we were watching a film and on her leg was a long red mark across her thigh I didn't want to assume it was self inflicted but after she left my house and logged into msn I asked her and she said she did it herself and then she said all the things I said why I did it. I told her to stop before it gets addictive and ruins everything and she was being like ''hmm I dunno I might stop'' and i asked her what she used and she responded with ''Anything sharp I can find'' naturally i got really worried and said she should get some help like I do if its serious and she said if i told anyone shed get really angry and hate me. I dont know what to do? why is she acting like this? and lately shes been talking on msn about how hard her life is and how she cant go a day without doing it. :/? And before anyone makes a nasty comment this isnt for attention I'm just worried about her