I Am a Christian and I Love Jesus

Hello fellow Christians. I've got a question for you. I've been raised my whole life that it isn't right to live together or sleep together before marriage. However, my best friend, my rock for all these past years, has suddenly changed her mind on this subject. She is also Christian and was raised with that same ideaology. But she's now living with her boyfriend and I'm certain they are sleeping together. Now before you jump on me and say that it's none of my business, I realize that. My problem isn't her...it's how I'm feeling. I'm sort of conflicted about it, especially since she's keeping it a secret from her family. I just can't resolve it in my head to where I feel comfortable with it. Am I a bad person? I've tried so hard to remain nonjudgmental but I can't ignore my beliefs either. I am an extremely tolerant and socially liberal Christian, for the most part. However, this is just one belief that I've held strongly. I am not planning on ratting her out or revealing my feelings about it. I know this is her business and whatever she believes is what she should follow. But how do I get over this, personally? I have a hard time letting go, and I want her to live a happy and healthy life...which she is. But I can't help but feel conflicted.