Why do friends do u wrong?

ok i have this friend and she went out with the guy i like but now she is starting to talk **** about me behind my back so i was talking to her and her bf which was the guy i liked is now mad ta me she mad up a fake forward saying her bf is cheating on her with me for 6 months and now idk wat to do i lost my bff which is the gry n she just kept saying i wrote that all i need help wt should i do?