Cheating.

My friend recently cheated on her boyfriend. She cheated on him earlier in the relationship when she was drunk but she cheated again with the same guy just yesterday sober. I've been in her boyfriends shoes. It started eating me up inside so I told him because I couldn't persuade her to tell him no matter how much I tried. My friend is really upset with me. Am I in the wrong for telling her boyfriend? I just feel bad about going behind her back even if it was the right thing to do