I don't understand people

I have no one I can tell this or complain to. There is not a single thing I can do to stop it from happenning!! I've been with my friend through three years of her relationship with such an amazing man! I've fallen in love with their love. They just seemed perfect together. They fed off each other. I helped them through all the difficulties. We all 3 became strong together and they formed a bond with me also. But how am I supposed to act when both of them are doing things wrong??? They have distance between them, yes, but that's always been the case! What am I supposed to do when he texts me upset saying he knows she's with another man. Am I supposed to lie to him just to protect THEM?! I want them to be together more than anything. We've all spent these three years planning our lives. We were going to move down with his family and get settled on our own, just me and her. And then after a few years they would get married and I would live by myself. Study in college and start a career down there... but now all this could change! What am I supposed to do?! I've been planning this for far to long to change now!! She is falling for another man... Someone I know is not good for her. Someone I despise. I hate him with her. I hate her with him. So what am I supposed to tell her REAL boyfriend about the other man?? If I told him everything I see going on, all her secrets, those two would be forever done for sure. But I don't want that. I want them together. But I hate lying to him. I feel so quilty inside. When I try and bring this up subbtly to her she freaks out and we get in a fight. I don't want to risk mine an her friendship... so I keep my mouth shut... But it's not right is it? I don't know what to do here and I'm getting so sick of everything! I don't understand her and it's making me so frustrated!!! Don't date two people. You just want the comfort I understand, but you've told me that I throw that "L" word around so easily... What do you think you're doing?!?! I'm just now finding out the truth about how many guys you tell! You purposly play with mens hearts, while I have to sit by and watch. And then you get upset because you get lonley, you don't have that person to lean back on if something goes wrong. You need back-up's... this is not how life should go and it's not fair to them or you. I know how much it hurts you. Deep down you have a good personality and you know what you're doing is wrong... MANY things you are doing is wrong... So why not stop?? I don't understand. I have to sit by and watch my friend get hurt time after time and her hurt others and not say anything or else it would result in my watching from the sidelines. If I said anything I'd be gone, like all of them. You make it hard for me to get close to you because I know how much you use people... You tell me I'm the only one in your life you can always count on but it doesn't stop me from being scared you'll throw me out like the rest of them...