Is my mom doing the right thing?

I have had a best friend since I was in third grade. I am now in tenth grade and my best friend recently tried smoking pot. She wrote me a note that night describing what it felt like. My mom was cleaning my room and found the note she had written me. Now she doesn't want me to ever see my best friend ever again. My mom knows how awful it has been for me to make new friends since she was remarried and made me move with her. I feel like the only person I am truly comfortable with is my best friend. And now my mom is trying to keep me from seeing her. I have been through so much with my best friend, and I know if my mom tries to keep me away from her it will not be better for either one of us. My best friend's dad recently ped and she suffers whenever she is at home. I am one of two friends she has told about her father's ping. We have been through everything together, from both of our parent's divorces, my dad moving to another state, her dad dying, her sister going into a mental institution and my mom getting remarried. She is the other part of me. Is my mom truly doing the best thing for me? If not, how do I change her mind?