Ok, here's the story. Well first off she isn't really my friend, more like a mortal enemy and everyone hates her and I figured someone should at least care a little bit even though she is a total b***h to me and my friends. (Her name is Abby just so we can avoid confusion because I have a lot of people in this story)
Anyway, I was hangin' out with my friend and she got a text from Abby (the girl who's in trouble) and she said she had a boyfriend who is 17 (she's 13) and we said prove it and she said she had a pic of his you-know-what and she said she'd send it to us. We said hell no don't send it. But... of course she did. She said she'd like touched it and stuff and that makes me wonder what kind of guy she's dating since he lets her take pics of his you-know-what and send it to people. And with the big age difference I'm worried even though we hate each other. Can anyone help me? Like give me advice on how to tell her she's in trouble? PLEASE ANSWER ASAP!

Instead of talking to her go straight to an adult,this is too big for you or your friends to deal with.

I could go into all the 'why's' it wrong,but you already know why,so my advice is,tell an adult what's going on and let them deal with it.

If you want to help her,that's what you do.