What should I do about my friend?

Now this is something I wouldn't really want to put on here but I don't know what to do anymore! So this kid in my youth group turned most of my friends at school against me by lying to them. I tried to protect his gf (my friend) from getting hurt again by him. He said I only said these things cause I'm obsessed with him when he isn't any good for her and flirts with every girl including me, my friends, and an older girl! The older girl and the boy are in my youth group and they now like each other when the boy still has a gf. I told the older girl what he did to turn my friends against me at school and she still chose him over me! I've known her for years now but she chose a guy who is too young for her and is cheating on his gf (my friend still)! I can't believe she would do something like this and she hates me now because the boy and I don't get along! What should I do?? I feel like I've lost a sister.