What should I tell my friend?

Ok so on her Ipod touch, I recommened a app that you can voye on pictures and talk to people through it if you wanted. I thought this was appropriate for her because I always go under the pet and animal column and answer questions on those which isnt bad at all. But I figured out that people can post questions like are you into me, or do you think I'm sexy and such. My friend is more immature than me and younger as well and when on her Ipod (she was watching tv and gave me permission to try out her apps.) so I went on that app I had suggested because she had posted a picture of herself as a profile picture and when I warned her she shouldn't post her picture she ignored me and I didn't know what to do so I decided to switch the profile picture with one of her dogs and told her how cute it was which may have very temperarly solved it. But then I saw she had a chat opened with the question titled: Are you in to girls? She isnt lesbian (she has a boyfriend) but she pretends to be lesbian and talks to her and they talk about masterbating and stuff and it ends having this 16 year old girl asking her if she wantrd to swap photos of the female private part. She agreed and thwy were going to do it on a different app and idk if she did... So this has gotten too far and I know her parents dont know but if I told them I could lose my friendship with her and she may do worse. What should I do? She doesn't know yet that I saw that... I want to talk to her about it and have her understand the seriousness of this but Idk how I can comfortably. What should I do about this?