AITA / I met my friend (female) in Year 7. We are now in year 11. In year 9, me and her stopped speaking since she always hit me, called me fat, annoying, social reject etc. But at the end of yr 9, at the end of the summer holidays, she reached out and we became good friends again. She has been dating this guy since year 9-10 ish (they broke up a once). But around 6 months ago, she told me she has been flirting with another guy while still with her boyfriend. Me and her were friends, but she only called me when nobody else was free, she would cancel our plans to see other people and she basically made me change myself because she thought I was embarrassing. Anyway, she was flirting with this guy for a while and meeting up with him. I told her to stop and to break it off with her bf or the guy. She didn't listen but eventually they stopped talking. But then, she started talking to another guy she knows. She was flirting, meeting up and making out with him. On valentines day, my "friend" had plans with her bf for valentines day but he canceled. So she invited the guy over and they made out and he SLEPT OVER! ON VALENTINES DAY! I kept telling her to end it with one of them but she wouldn't. So, I told her boyfriend that she was cheating. I gave him the small proof I had from mine and my friends snapchat chat. Am I the asshole for telling him?