Need help with bestfriend?

my best friend is 12 and I'm 13 and she's started hanging out with these heavy metal people at school. I really don't mind her doing it but they are all like year 11 and stuff.. And ever since she's been with them she swears all the time and plays metal on her phone whenever school finishes and it can be embarrasing because she'll go completely mental and start running round. She acts like a 5 year old :/ And she's still my best friend but I've grown older and I really don't like running around like she does, or having no shame whatsoever like she does. Plus she phoned me up today nearly crying because she'd "accidently" dropped a knife on her wrist or something and was really scared her mum would think she's self harming. And she said it was a tiny cut, and it was a crafting knife she dropped on it. But I'm worried that she's will start harming herself to impress all these people she hangs out with because some of them harm themselfs too, and she already acts, dresses and listens to the same music as them.It's just weird because one moment she was listening to girly music and the next she's dressing in black and hanging out with all these people.. I'm just worried because 1. She'll get made fun of (she already does tbh) 2. It's so embarrasing to be with her even those she's my best friend and 3. she'll start harming herself to fit in.. I know for a fact she is the happiest person EVER but she makes friends with anyone and will do anything to make them like her. She dosne't really have any commen sense, I don't know what to do :S Help?