My best friend is in a relationship with a very dodgy guy... what should I do?

My friend and I are 16, and she has just started a relationship with a guy who is 20 - which I guess can sometimes be alright, although I think this guy is really dodgy. He has lots of drunken pictures on Facebook, and he's possibly into drugs as well. She met him through her scout group, and I'm really concerned that she's going to get hurt in this relationship - however, I acknowledge the fact that her relationship shouldn't have anything to do with me. I'm worried that he's just going to use her and perhaps pressure her into drugs and maybe sex... please tell me whether I'm overreacting and that I should sit back and be happy for her, or if I should be worried and either keep quiet or have a talk with her?