My friend is in a bad situation?

This is quite a long story, and when I asked one of my other friends for advice, she didn't believe me. But I promise that it's true! So basically, il start from the beginning.

We were out shopping, me, my friend Amie and my friend Emily, and this boy came up to Emily and started being flirty, and she was kind of doing it back. Then he started feeling her top and stuff, and pushed her really hard against the wall. She let him kiss her, and while they were standing there making out me and amie found it really awkward (this was in the middle of a shopping centre) but then Emily came over to us and told us to meet her later outside, which we were a bit unsure about but she didn't exactly give us a choice. Then about an hour later she rang me saying she would be later, but there was loads of noise In the background.

Later when we met up Emily looked really rough (her hair was all messed up, and she like smelled really weird :S ) and when we asked her what had happened she just smiled and said nothing, which it obviously wasn't.. Later she let it slip that they'd had really rough sex, and they didn't use protection, AND she was really jittery and stuff so I think she may have used drugs which worries me loads because she's only 14, and the guy looked a lot older...

But yeah, then when we went to school a couple of weeks later me and her were talking about it, thinking we were alone but our head of year heard the end of it, when I was saying how she could get hurt and how could she trust this guy, so the next day my head of year was asking me what's wrong with Emily cause she's acting really strange and stuff, and I ended up telling her about the sex but not the drugs, and now Emily refuses to stop totally, she's acting so rude and a couple of times she's gone missing at lunch time then come back smelling like she did on that day.

I know this is long but I wanted to say the entire story so you'd understand more, and now I'm really worried about her, what do I do?