What do I do? To Tell Or Not To Tell...?

I’ve had with my friend boyfriend/fiancée. They’ve been engaged for 3 years(not like that matters at all) the first time it happened we were all drinking and laughing, having a good time. She pasted out then it was me and him he tried to kiss me I leaned back, but he caught me. I told him I forgave him, that I wouldn’t say anything to her cause he was drunk. Then he touched me, he said “you ganna tell her about that?” and we had . I don’t know why I couldn’t stop myself. It was wrong and I know that, that’s not me. That’s not what I do. When this started we did it a lot. Now, not so much. The is amazing, best I’ve ever had in my life. He told me he loves that look of shame I get on my face every time. I feel horrible after every time, but every time he threatens to tell her. Its strictly between use and we both know it, nothing will ever come of this nor would I want it to, he’s a jerk. But I’d be lying if I said I didn’t secretly want it, a little. I don’t ask for it, I never would. It doesn’t happen much anymore but once in a while he’ll come to me. The thing is though that I jonez for him, like a drug addict. I can’t stop thinking about it, its always there. I feel like I need a fix, bad. But here’s the thing now, I’m friends with both of them. Do I tell her or not? If I tell her I’ll not only will I end our friendship but I ruin her relationship, and for what. To clear my conscience, to make myself feel better? Or do I tell her because he’s probably likely to do it with others as well. What do I do, I feel horrible. I don’t want her to get hurt, she’s like my sister. Which makes I worse. Please, please don’t judge me to harshly. I hate this.