Should i tell anybody what my best friends doing?

im a 15 year old girl (im an emo tomboy. but i dont cut myself) my best friend is also 15 years old she was a really happy person last year. we called her rainbow girl because she used to dress in so many colors and patterns she loved the nickname. towards the end of our 10th grade year she started changing. she started wearing makeup. (all black makeup) then she started dressing in all black like emo cloths. she soon dyed her hair black and cut it up to her shoulders. she got a piercing on her nose (her parents let her) she wears black converses. she became depressed. i saw her today. she was wearing a black hoodie and hiding her arm. i said let me see your arm. and she begged me not to look but i took her arm and looked and she had cuts all up and down her arm. and i said what the hell are you trying to kill yourself. and she said i dont know , nobody loves me anymore and she told me everything thats happening. and i said i love you! and i kissed her and she kissed me back and we made out for a few minutes and i said dont do it please i love you ill be your girlfriend we can be s. and she said she wont do it but shes sorry but i dont know if i should tell. the cuts looked old. but didnt look near her wrists. its been a week and she hasnt done anything. sometimes she comes over and talks to me whats going on and tells me she isnt going to do anything but im really scared still. i dont know im just worried about her.