I (16-18F) and my friend (16-18F) are very close. I however moved away from where I met her last year. She still lives there with our mutual friends, and the subject of this post. I'm going to call her Sun for clarity.
Around 5 months ago Sun had made a joke on a catch up phone call with me about having a crush on her extracurricular activity teacher. No biggie, simply because we're young and it's common for teen girls to find older men cute in an innocent way.
Fast forward to about a month ago. My friend who keeps secrets very well told me she needed to tell me something very serious. She calls me, and tells me that that activity teacher "confessed" his mutual feelings for Sun, and that she told me because of how wrong that felt and I agreed.
A week after that I found out something that has left me sick. My friend told me it has escalated tremendously. Sun and this man are in a "relationship" (I don't even want to call it that because of the nature) They frequently hang out and are dating. At this time, Sun had only told my friend, and one other girl. I was disappointed in my friend for not reporting it to someone, or telling someone who could do something because I am so far away from them. They were not taking it seriously and at this time Sun did not know I knew yet.
Now yesterday, I called Sun after us not calling each other in awhile. She had told me she needed to tell me something and I had figured it was about him. She tells me and I begin to give her a gentle reality check. I did not want to be harsh because she is being manipulated and I was sure being harsh would not help. I told her the reality of it. Then after a few minutes of talking about it she tells me that her close friend and our mutual friend whom I will call C, does not know because she was scared that C would tell her parents. (For context C is a very smart and mature girl with a great head on her shoulders, and she would heavily disapprove of this and would take the action I am unable to take due to my distance.)
I was shocked by this. She asked me not to tell anyone, however I feel as if I must. I told my friend if no one does anything I am going to tell C, so that Sun's parents can find out and this can be solved in the manner it should be. I have threatened to tell C her secret and will potentially be betraying everyone's trust, however it is such a serious situation that I think needs to be taken care of. Advice? (for further clarity the guy is 21)